I just measured mine on my Forest River Wildwood trailer
The wood paneling on each side is 1/8" thick and the inner space between the paneling is 1-1/2" for a total
of 1-3/4 "
A suggestion which I did on my last trailer which worked great.
I made backing plates for towel racks, a toilet paper holder and a few other items using half inch oak and then routing the edges, staining it and finishing it with polyurathane.
In each one I used two brass screws with sharp points and just long enough to penetrate the exterior paneling. Then I coated the backs with glue and putting pressure on the screw driver screwed them in place, (the screws just act as clamps). The next day, I would screw my item to the backing plate. In eight years with the trailer we never had one come loose and had quite a few comments on how attractive they were
Jack L