I've also got a 1/2" thick plywood belly on it for piece of mind when loading/unloading..... For future builds (especially smaller campers) I intend to use 4mm ply and glass the entire thing.
Larger scale builds, Im still on the fence. Though Im leaning the way of keeping the 6mm ply and glassing the entire thing.
It's tough to get this stuff right. Baring a lot of experimenting and detailed structural analysis, we are just guessing. Hopefully semi educated guesses though!
16 years ago when a built a similar camper (under time duress and no internet and minimal budget in someone elses garage) I worried about a lot of things and just hoped for the best. I initially was going to paint it but decided to fiberglass at the last minute. My previous camper built with similar constraints was painted with the wrong paint (and had a sunroof! bad idea) and ended up rotting. I figured better paint and design would fix that, but fiberglass would be an even better choice. I'd never glassed anything in my life, but it wasn't hard, even with polyester resin and widely varying temperature. On some parts around door and window frames I merely painted the resin over the wood and it held up great. Trying for a smooth surface with fiberglass is a lot of annoying (itchy) work, but if you leave it textured, it isn't bad at all. No sanding or cutting of glass, except a little on the edges when the tape is applied after assembly. I built the panels and glassed them first, so I could work horizontally, then did the edges after joining. The front piece was curved so I had to do vertical there, but it wasn't that hard. I used thin 2.7mm luan skins for walls and top, but they were better quality than they are now. Anyway, the fiberglass over luan was super tough. The fairly thin but rock hard skin seemed to add a lot to the durability.
The foam I used in the panels was crap. Really cheap and weak EPS. I'm guessing XPS wasn't around or I didn't know about it. Based on my prior experience of similar EPS coming debonded from the face sheets fairly quickly, I used a lot more stringers in this build, 12-16" centers.
One thing I worried about quite a lot was the floor. Like now, I mounted the camper directly to the truck frame, only on my '84 Toyota there were only 4 points. I found some steel "hat" pieces used to clamp 2x4s that were very heavy duty (I'd guess 3/16 thick) and screwed and glued those to the floor, and mounted them with bolts to frame. No cushioning, no bushings, hard mounted. That floor was 1.5" core and 5mm luan faces, with mostly 2x2 stringers, with 2x4 under the mounts.
The end result was that I put ~45k miles in 3 years on that camper before I had to give it up. Lots of bounding over washboard and rocks, bashing through trees branches, and sitting out in the weather 24/7. No leaks and no problems whatsoever. The mounts held and the lack of isolation from the frame didn't seem to hurt the camper. No evidence of the foam failing or debonding, but the stringer spacing was tight enough that I'm not sure I would have been able to tell.
One thing that concerns me now is that I have more time to plan and build, internet access, more money, and my own garage!... but I'll still screw something up that is hard to fix. There is a fair amount of luck involved I think.
? If you are doing fiberglass over foam, I'd consider doing a textured final layer like Monstaliner. Trying for a smooth shiny surface would be a lot of work, fairing and sanding.