I got the perfect spot in my off-road POPUP trailer inside one of the cabinets floor. Safe would be under the trailer floor with access inside a floor cabinet.
Another good spot might be under the storage area of the pull up bed.
Also in the bed room of my fifth wheel is a sink cabinet with a hole that goes into the pass-through storage area for dropping dirty clothes thru into a basket. That might be a good place for a floor mounted safe
We almost would rather not really use a safe for real valuables and just keep them in the truck cab when on the road.
Having a lock box in the trailer would have its value I guess.
Some think they might want to store a house firearm there but by the time you got it opened up to get to your firearm would probably take too much time... Mine hangs in its holster next to the front door covered with a jacket...
I saw one once in the kitchen area mounted in a door looking fake microwave unit. Thought that was neat...
Roy Ken