The walls on my 18LT Work and play are 3/4 inch plywood, with about 1/32 inch of fiberglass laminate on the outside. The bolts used to mount the TV, really the nuts, are rubber covered on the outside, to seal against water. Same nuts used to mount the beds, etc.
If in doubt, and you have a toy hauler, look at the side vents. I got a good view of my walls, and thickness there.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT