My first camper was a 2000 Lance 1170 that came with my truck. After buying it we were cleaning it and I happened to put my hand through the wall in the overhead area. Further inspection found dry rot all over the place in the area. Took it to a local repair shop, spent $3000 for them to rebuild the front area and thought I was good to go but later found additional rot and leaking in other areas. Feeling rather duped by the seller, I had enough and sold the camper to my cousin (full disclosure of the issues) and moved on.
I then looked for a Bigfoot Camper knowing that my intentions were to be on the road a lot and I wanted to avoid anything with seems or slides to reduce the chance of water penetration. I bought my current camper nearly sight unseen and I have not had a single issue of water penetration yet. My only complaint was my 2001 Bigfoot 10.6E was before they used stainless steel fasteners on the exterior so I have had to replace the rusted out ones as I get a chance.
Getting back to topic... even in my fiberglass camper, there is still a plywood backing and interior wood that I get concerned about. One area of concern is where the fridge vents through the roof, the plywood is exposed (as the hole is cut through the structure) and they did nothing to treat the exposed wood. It's "protected" by the fridge vent cap but humidity and water penetration are still possible. I've considered sealing it with some sort of black tar sealant or something but haven't gotten there yet.
I am also looking at upgrading my solar panel to something larger and will be reaching out to a fiberglass shop to have them glass some mounting posts onto my roof with a way to bolt to them rather than to screw through the roof and hope the additional holes won't leak.
I think the bottom line is that wood in the interior will last just fine as long as the exterior is well sealed and kept that way. To this end, pay to have your roof resealed every couple of years and if possible, store inside.