2025 GMC sierra 1500 4x4 towed behind motorhome

I wouldn't use the switch on your ground or positive, I would just carry a 1/2" wrench in the truck and take 5 seconds and physically disconnect the ground from the battery.  if that switch fails while your driving you are kind of screwed, and like I said it take a few seconds to loosen a ground off.  

Based on the questions, I’d question your ability or experience to do this, but…
To answer your questions, rather than ask the questions before even verifying what you’re asking, why not actually run the procedure and it will answer your questions. 
PS, you don’t actually have to tow it down the road to verify what you’re asking. And as a side benefit, you’ll learn something rather than being told it and likely understanding less. 
Also yes, trans in park is the right way for reasons mentioned above.  There is no magic to disconnecting the battery. Pick one or the other posts to disconnect. Neg is best for reasons you can look up. 
Don’t be surprised when you have to redo any pre set electronic things. 
Any universal battery disconnect that fits the battery post will work although new vehicles have cables generally routed “perfect” with little play and semi vehicle specific connections, so may have to move some stuff around or modify the battery connection. 

Dan, In the manual there are actually 13 steps in setting up your truck for towing 4 wheels down and 12 steps for disconnecting it. I would suggest you make a cheat sheet for both and keep it in the truck to refer to. Skipping any step could result in an expensive repair bill.

(1) There is no reason you can't use that switch on the ground side. Buy it, install it on ground at battery, and enjoy. You're welcome.

(2) Vehicle MUST be in park and transfer case in neutral at all times while moving. I know a guy who failed to do that, left transmission in neutral as well, and his reward was needing a new transmission after the first such tow. The reason being that with the fluid couplings in torque converter (and elsewhere) there was just enough rotational force transferred to cause the transmission internals to rotate, albeit slowly, but without proper lube since the transmission fluid pump wasn't being rotated, that it burned up all the bearings in the transmission. Granted it wasn't a 2025 or a GMC, but the design of things and instructions were the same. 

(3) And yes, the truck stays in ACC mode the whole trip to keep the steering from locking up.