Ehh looks like my youtube links ruin the formatting. Ohh well, y'all will live
Thanks for the Everlander links and other info! Just some notes on your build...
The challenge is a good bond to the foam... not the FRP (probably). My experience with epoxy isn't that great. It works it just isn't hard to pull the skin away, but that may be due to the foam being weak more than anything. I tried PLPremium with plywood skins also with similar results. I suggest checking out the Styromax site and videos. As I recall they mention the adhesive they use. One thing to also note is that the XPS designed for panel building has grooves for glue.
Surfboard makers have had issue with gas being released from the XPS at elevated temperatures causing delamination. There is also an older build on this forum where a guy did wet layup on XPS and he got delamination bubbles on the roof the first time he pulled it out into the sun!
Anyway, I knew all this 3 years ago and still built panels using 25psi Foamular. I dealt with the possible issues by edging all the foam pieces with wood to give it more structural integrity and also provide interior hard points, and planned to have an all white exterior to minimize solar heating. The PVC foam will not need this, which will simplify building... I hope.
I made some samples with the PVC foam and carbon and FG layup to test strength and stiffness. I made a spreadsheet to see what to expect. Narrow 48"x4" strip that was 3/4" thick supported on the ends and weighted in the center. With 0.8mm woven carbon/epoxy skins, no problem supporting my weight (180lb) with ~.8" deflection. Woven FG of the same thickness deflected ~2.5x as much which is what I expected. FG of similar consistency is nearly as strong (load before breaking) as carbon, but much less stiff. And it's a bit heavier because carbon is a lighter material.
Then I broke both of them by dropping a big (~50lb) rock on them... doh.
Before I bought the PVC foam I made similar samples with XPS. I marveled at how stiff the carbon was vs FG; the difference was very obvious. I gave it to my 120lb wife to feel the difference, and she promptly busted it by pressing with her hand! Granted she pushed pretty hard, but nothing like my 180lb weight. The foam buckled on the compression side and carbon snapped. Then I took the FG piece and loaded it with a mere 36lb over a 30" span. It deflected about what I expected but then kept going (creep)... and after a few minutes it snapped in the same manner.
My other "scientific" testing is building small samples and whacking on them with a hammer. As expected the carbon seems more likely to crack from this abuse, but only after a few hits in the same spot, probably from the foam compressing underneath. The FG is obviously compromised but the cracks don't seem to go all the way through as readily. Again, there is a massive difference between XPS and PVC... the XPS falls apart easily.
I don't know if you are considering honeycomb panels, but CarbonCore also sells them. As I recall ~$280 for a 1.5" sheet. Do you know where Everlanders sourced theirs?
It's easy to overthink this... but then I hate building stuff and having to do it over. I made a frame mounted camper 20 years ago... in 2 months, under duress in a friend's garage (they wanted me done in 1 month!). I had little knowledge, no one to advise me, no internet to consult, no plans, etc. I made it out of 1x2s with with crappy EPS foam in the middle and 2.7mm luan skins on both sides, then coated the exterior with FG matt and gelcoat. Very glad I used FG on the outside... it was bomber. I bolted the camper straight to the frame ('84 Toyota). It survived thousands of miles of offroad abuse with no issues. Wood is a fine material, you just need to keep water from getting to it. Check out Idasho's build if you think you'd like to do something similar. Really the only reason I'm doing this one different is that I always like to do new things... plus I'm not as poor now, so I can spend more money. I have trouble deciding though, because I'm really not in a hurry...