Battery Disconnect Switch

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.

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.