You should install the switch on the ground side of the battery,  and it is a lot easier to do. 

I used to work on boats which are essentially RVs on water.....never saw 12 volt shut off use the ground always used the positive.  I suspect the worry is it would be easier to accidentally ground the system...not sure.  The circuit breaker I used is a 40 amp and the one I got easily fit in the battery box and is easily resettable.  I used the circuit breaker because there is no protection between the + side of battery to switch.

Originally posted by Toyanvil <font face="Times New Roman, Times, serif" size="3">I will just add this, I do know about <span style=": rgb251, 251, 253;">Article 551 and it is for the 120 AC in RV's. Here's a link you should read before installing on the positive side. It is written by </span><em style="-sizing: border-; color: rgb17, 17, 17;">Mike Sokol who is an electrical and professional sound expert with 40+ years in the industry. here's his book<span style="-sizing: border-;"> RV Electrical Safety</span>  For more info on Mike’s qualifications as an electrical expert,<em style="-sizing: border-; color: rgb17, 17, 17; font-family: " Sans"; font-size: 14px;">  http://www.rvtravel.com/rv-electricity-12-volt-tech-battery-disconnect/

Yes NEC Article 551 discusses ac circuits. But that does not mean the NEC does not cover dc circuits. That is simply incorrect. Article 690 covers solar which is dc and article 480 covers battery installations, which are obviously also dc. In fact essentially all of the NEC is applicable to dc as well as ac systems. All the sections covering grounding, fuse and circuit breaker selection, conductor ampacity are equally applicable and need to be followed say for an off grid home to pass an electrical inspection.

The reason it is better to first manually connect and disconnect the bolted battery negative terminal for maintenance purposes is because you can’t inadvertently short your tools to ground if you disconnect that first, then the positive. That maintenance activity has nothing to do with where you install a permanent switch like a battery disconnect.

The article you are referring to claims that placing the disconnect in either positive or negative works “equally well”. I disagree. Every other switch, fuse, and circuit breaker in the trailer is in the positive leg, because FR is following the Code and good practice. That way you know that the grounded conductor (negative) is solid and any open circuit is in the positive leg. Why should the disconnect switch be any different?

No explanation other than there might be a fuse block in the positive line which could make disconnect switch placement there inconvenient is given a a reason for placing the disconnect in the negative line. That is certainly not the case in an rpod, and even if it was inconvenience is not an excuse for violating code. I’d love to be a fly on the wall if an electrician tried that in on his local inspector. 😜

As for installing the switch, you can also just cut the existing positive conductor and install an inline switch if you have enough slack and you want to do it that way. Pedward’s suggestion to use a circuit breaker is a good one, that also will protect the conductor from the battery to the panel. Any 12V taters 40A breaker should work but do gets manual resettable one so you’re not waiting around for an automatic one to decide to reset.

ordered, thanks!  

I was just going by how all the boats i delt with were wired as all the larger ones have battery switches and they are always wired from the positive terminal......I just figured a TT would be similar.