A hollow cavity like that is common and may or may not have 120 or 12 volt wiring or low voltage wiring (like for tank sensors) in it. The cavity is usually 3/4" to 1" thick.
If you saw all the wiring in a ceiling cavity and all the sharp-pointed screws they put into the ceilings, you'd be totally shocked... I know I was the first time I discovered that. Not sure what size of screws you're using - #8? You *could* screw into the 1/8" luan plywood and then remove the screws and file or grind the sharp points off.
A #8 screw doesn't have a lot of holding power in the thin and relatively soft luan plywood they use in RVs. I actually had that happen today on a thingy for one of the pleated blinds and had to move it slightly and put the screw back into solid luan again.
Instead of screws, you can also use Oscar rivets. They are a type of rivet specifically made for applications like this in RVs. Not cheap but if you only need a few you're okay. They are very strong too and have way better holding power compared to screws and won't hurt existing wiring. For paper towel holders, towel racks, TP holders and the like, Oscar rivets are a good choice.
If you ever want to add recessed puck lights, a receptacle, switch or whatever, those hollow cavities sure come in handy!
