Are you talking about sanding the smooth, gelcoated parts of the trailer, like the outside of the closet/cabinets? Or the inside walls (normally a more "globby" type surface that was not molded)?
It sounds like you mean the walls, but I wasn't sure because I didn't know they made any with molded gelcoat walls.
I would start by reading the instructions on the paint
or primer you are going to use. They will typically recommend which grit you should stop at (usually something in the 150-220 range, but that could vary).
I would probably start with the 120, but I wouldn't choose until after finding out what my particular paint
instructions called for.
80 would be the absolute roughest I would start with, but if you do, be careful. And it's probably too rough for what you are doing (but I can't see your walls).
use really rough grits on fiberglass (I've used 36 many times), but that's usually when you are trying
to remove material in preparation for fiberglassing - when you want to make a bevel. It's quite possible to sand right on through.
Because our trailers are relatively thin, and assuming your interior is not smooth molded gelcoat but is the rougher, "backside," like mine is (I can see it inside the closet even though my walls and ceiling are covered with Ensolite), I would almost be tempted to use something more flexible and to sand by hand, so as not to remove much material. So just be careful with the power sander until you get a feel for it.