There is a reason ALL mfrs place the battery(ies) on the trailer tongue. I would not try to second guess them and place it inside the trailer.
Just as a data point: I believe the 13' U-haul
trailers (I have a friend with one) put their battery inside under the "U" part of the dinette/bed in the rear of the trailer, with a venting hose leading outside.
Sailboats often have batteries mounted in either the living space or the engine room, as there are limited options "outside" (i.e. above-decks)
. Attention is paid to battery boxes, hold-downs, and venting.
I'm not necessarily advocating putting batteries inside as a preferable location, but I think it has been done. Given my experience with extra tongue weight
on a trailer that does not even have a battery (yet), I will probably consider creating a sealed/vented locker under the dinette or elsewhere for one/two batteries.