It's usually best to interrupt the positive cable, (red if you have red and black, or black if you have white and black).

The switch can be installed in the battery box if there is enough room, or inside the trailer near the converter where the positive cable enters the trailer.

Installing the switch at the battery box up front means the battery will be offline for maintenance charging if hooked up to shore power and the switch is off. Or, if hooked up to shore power, turn the switch ON.

But if you plan to leave the trailer NOT hooked up to shore power when stored, this is a good option, as it removes all drain from the trailer electronics during storage.

The red vs. black wire thing can be confusing, just find the positive (plus) battery terminal and connect it between that wire and the battery post.  

As podwerks says, you should put the disconnect in the positive line. The negative is a grounded neutral so shouldn't have any breaks made in it. 

Personally, I disagree with having it on the positive side. Please read this, then tell me I am wrong for putting the switch on the negative side (and disconnecting negative first, connecting negative last)

https://www.popularmechanics.com/cars/how-to/a1454/4213127/ - https://www.popularmechanics.com/cars/how-to/a1454/4213127/

You're free to disagree and wire yours the way you feel it should be done...you are not 'wrong'...but I am also free to recommend 'best practice'.  

The problem with switching only the negative side in a battery disconnect switch installation (in a trailer) is that now, even with the switch turned off, the entire hot side cable still has 12v battery potential on it, all the way to the converter and other electronics inside the trailer.  If there is a short-to-ground failure in any of those components that have a ground connection, (and most do) then now we have a completed (but probably inadequate) ground, which might 'liven' things up more than we would like. 

The differentiation is between permanently installed disconnects and maintenance activities. The national electric code only allows grounded (neutral) conductors to be disconnected for maintenance or under special circumstances that we don't need to go into here. 

We should all follow the Code when wiring our trailers. If you look at your home wiring you'll see that the white (neutral) conductor is never disconnected or fused. The colored conductors are always protected by breakers or fuses and are the ones that are switched. By following that practice you can be assured that if you have a fault somewhere you won't end up with more current flowing through an unprotected conductor than its rated for. 

I think you probably answered your own question.  

Obviously you'll want the Victron display inside. The shunt has a pc board attached so that should be protected from moisture and temperature extremes as well. So, I'd suggest placing the shunt  in the negative battery connection near the charger and the disconnect either at the same location but in the positive battery connection or outside on the battery positive terminal. 

I get your concern. Since it looks from their literature that Victron wants their shunt in the negative leg you'll need to be sure that there isn't more than one chassis ground return to the battery. Otherwise the shunt will only pick up part of the current. 

I've never looked at that so I can't say for certain. Maybe someone else has?

One way to check would be to temporarily lift the negative connection from the converter which I think goes to the frame under that area and see if any items are still energized. If you have one you can also try a dc clamp on multimeter to see if there's still any current flow while that connection is lifted.

If this installation is beyond your knowledge and comfort level, I would suggest you take the trailer to a local RV shop or small mom-n-pop auto shop. Or if you have a relative or friend who is handy with tools and electrical repairs, they might do it for you for a pizza and a 6-pack of beer.

I can easily do this stuff, but some things, like welding, and roofing, I pay for.

The consequences of an error might cost a LOT more than the installation fee you end up paying.

I have a wrench in my tool box that has a divot taken out of it from before I learned to take the negative terminal off first. It did not take long to learn that lesson. The wrench was instantly hot.

Yes the NEC applies to both AC and DC circuits. There is no difference with respect to grounding practice and overcurrent protection (other than you don't call the hot conductor the positive in ac circuits, because the voltage alternates positive and negative).  I've designed hundreds of dc power systems and all were required to meet the NEC. 

No one is saying for you not to lift your negative first when doing battery maintenance.  Then, proceed to lift the positive as well.  No one wants to be melting wrenches. I always use one with an insulated handle when I mess with batteries because you can easily end up shorting across the battery terminals as well as to ground. 

But the switch is permanently installed. it's going to be left open indefinitely and unattended.  Podwerks did a great job I think explaining why that should be in the positive (ungrounded) leg. There is no complete circuit until there is a fault or inadvertent connection in the system somewhere. That's what much of the electric code is for, protecting your electrical system in the event of fault conditions, not from normal conditions.  To accomplish that the code provides for a set of neutral conductors  which are not switched and are always referenced to ground and another set of hot conductors that have the necessary fuses, circuit breakers and switches in them.  

"If this installation is beyond your knowledge and comfort level, I would suggest you take the trailer to a local RV shop or small mom-n-pop auto shop. Or if you have a relative or friend who is handy with tools and electrical repairs, they might do it for you for a pizza and a 6-pack of beer."

Excellent advice, Podwerkz. I was able to get it done relatively quickly once the concept of interrupting the circuit sank into my brain. Works like a charm. AND i put the switch on the positive side. Thanks for all the input. Much appreciated.

It depends where you live. What you are saying is valid for cold winter climates but not warm ones. Cold is good for batteries because the self discharge rate is greatly reduced. They won't freeze if you fully charge them first.  

But, if you do that in a place with warm winters the batteries will discharge and the electrolyte will most likely stratify, resulting in plate sulfation and either reduced capacity or dead batteries.

So, if you live in a place with warm winters, leave the batteries and the charger connected and turned on if you can. You can't go wrong doing that unless you forget to keep water in them. Since I live in a relatively warm climate I don't have a disconnect switch and wouldn't use it if I did. 

If you can't keep them charged via the charger where you store the rpod then remove them and trickle charge them at home, or get a small solar module and maintain their charge with that. 

In my case, the switch breaks the positive connection from the battery to the trailer, inside the trailer. But the positive wire up to that point IS energized, even with the switch off. This way the solar controller can continuously feed both positive and negative. This falls under 'best practice'. Just like using red for positive and black for negative. The electrons sure as heck don't care what color the wire is, but WE need to try to be consistent, using 'best practice'.

Out on the trailer tongue, IF the battery switch is physically mounted there, such as in a metal battery box, it is less critical which wire is interrupted for switching: IF there is no (and will never be a) roof mounted solar panel, or solar controller inside, and you can GUARANTEE that NO ONE will ever hook up a standard automotive battery charger to the trailer trying to manually charge the batteries, this person having no idea about the switch, and clamps the red clamp to the positive battery terminal, and the negative clamp to the trailer frame. If they don't see or know about the negative acting switch, they will get no results...no charging when they hook things up.

In other words, future proofing. 

Best practice, among other things, stays consistent: what you do today, and what you want to add tomorrow, or next year, and what a random RV technician will know about, and poor Cousin Charlie, who buys your trailer after you die, and has to figure out what the heck is going on...and what was added, and why was it done that way. 

Ok, once they get it figured out, fine. 

If you chose to completely re-wire an RV using pink wire for ground wiring and green for all the positives, it will work. But no one else who works on the wiring in the future will know what is going on, because how could they?

But 'best practice' can usually be figured out much quicker, because most of the time, it's also common sense...it is done the same as all other common wiring is done. Common practice and best practice are often the same thing. 

Disclaimer: Not ALWAYS 'just' common sense, because often there is solid engineering behind it.

You might inadvertently energize the circuits from the tow vehicle or the charger for example. That could create a surprise when you reconnected the battery, or a fault in one of the neutrals could create a fire hazard since they don't have any over current  protection. In the rare instances when there is a real need to float (no ground connection) an electrical system (typically for some kind of specialized industrial equipment) both conductors are treated as hots and the code requires both conductors to have fuses or circuit breakers for that reason. 

Yes, consistency is important. Its much easier to stay safe and to troubleshoot problems efficiently if you go into it knowing what to expect. If your system is installed per code  and you see a white wire, it will be a neutral and will always be connected and grounded. A colored wire will be either hot or off depending on the switch positions or whether the fuses are in on not. 

Turning the conversation around, I don't see any benefit in not following the code by putting the switch in the negative leg. its not like opening the switch is going to risk shorting a tool to the frame, right? 

That's a great way to do it. 

The disconnect is in the negative leg: WRONG.

There is no indication i could see that he has any fuses or circuit breakers to protect the cnductors to either his trailer or his solar array: WRONG.

Just because somebody puts a video up on youtube doesn't mean they know what they're doing.