I have mounted a bunch of stuff to the walls in our TT and use two methods;
1. For heavy objects. I'll push in on the wall at various places until I find an area that is solid and has no give. Then I drill a small hole about a half inch deep to make sure there is solid wood.
I make sure that where I drill it the hole will be hidden behind the object I am mounting, just in case my hole misses solid wood.
Once I have areas that I know have solid wood, I use screws that are no longer then half inch, and I coat the back of the thing to be mounted with a good grade of glue.
2. For light weight objects,(between where there is a solid wood backing) I coat the objects back with a good grade glue and then just use short, half inch or less screws.
These screws just act as clamps to hold the object until the glue hardens and secures them
I the ten years that I have had our two trailers, I have never had an item fall off the wall.
The heaviest items I have installed so far have been a large medicine cabinet in the bathroom, a 16" x 16" kitchen cabinet and an eight foot long clothes rack in the bed room. Many times the clothes rack is loaded with clothes for our thousand mile trip to and from Florida.
For what it is worth, I have found that there is solid wood around the entire perimeter where the walls meet the ceiling. It extends down about an inch..
Jack L
Jack & Nanci