FWC uses aluminum tube framework from your bed rails up. The lower portion of the camper, bed rails and below, is made of wood. This isn't cheap pressed particle junk though. There are some minor exceptions but that's the general recipe as far back as 20 years ago, probably longer as others have said. Their aluminum frame is really where I consider their reputation to come from. It's quiet often lighter, and definitely stronger, than most of the long standing slide in companies. Some of the newcomers are doing some cool stuff with composite materials but they tend to be more expensive for that reason.
Herbie is spot on. The older lift panels were made of wood and wear out over time. They can be replaced with the current composite panels but FWC need specific measurements and it's still a bit of a project. Use that as a bargaining chip if those are worn out at all.
I've never seen any issues with the lower wood construction unless it came from user error/destruction. Keep those turnbuckles tight. I'd also give the vinyl top a very good look over. Some of the older campers used different pop up fabric and it's a major project to replace if you have issues.
Electrical can be pretty easily upgraded, siding can be patched or repaired. If you find one with fiberglass siding, look it over for cracks. Check the underside of the bed area, on the exterior, that is also a laminated wood.
Ask the seller to pull the camper from the truck if it's still installed. I'd still want to see the camper from below before purchasing second hand. Check the anchor points in the wood base, make sure there is no rot like you're referring to. I've seen 20+ year old campers in great condition if they were taken care of. I've also seen three year old campers totaled because people don't check their turnbuckles. Buy a fresh set of turnbuckles and keep the old ones as spares.
Feel free to reach out if you have specific questions.
Good luck on the hunt.
