There is no point to bother with a "fixer-upper" water damage project, unless someone is just really determined to do it. The value of the rig is still going to be valued as scrap.
The only towable that I would even consider a complete inside refurbish on would be an Airstream due to the fact that people have good success at tearing those down to the metal skin, then building those up. That is still pretty costly -- a firm in Austin is asking $12,000+ for a refurb, and considerably more for a true restore (with vintage seats and appliances.)
There are other trailers that are made completely out of aluminum, inside and out. Those tend to go light on wood, so anything rotted can be replaced. Livin' Lite comes to mind.
Major water damage on virtually anything else is best solved by writing off the losses, parting out anything usable (refrigerator, water heater), and then having it go to its final destination at the scrap heap.
Is it hard or costly?
I would have to say the answer to both is - not really.
But, these projects tend to be massive. You have to be prepared to repair much more than any original estimate. Even if the damage is only as much as you can find in the original survey, there will be other things that will require attention just so the repair alone doesn't look stupid.
If you are a capable wood worker, and can work a plan, it can be done. Don't take on a "fixerupper" or (as labeled in Michigan) a "Hunter's Special" to save on the purchase price. Yes, the work won't be all that hard and the materials not that expensive, but the time will be weeks and months, not hours.
Want to learn a lot about old trailers at once. Go to the Tin Can Tourist Gathering at Camp Dearborn (just east of Milford) on the last weekend in September. There will be more then an hundred rigs there. Lots of people to talk to that all like to talk about what they had to do. It will cost 6$ to drive in and well worth the trip.
I had a small Class C with water damage. It was a Cayo (Avion before Avion), so it was all metal structure, but the plywood floor was rotted and some of the otherwise beautiful cabinetry was damaged. I would only tackle such a project again if the vehicle had some sincere value that made the effort worth the commitment.
Matt & Mary Colie
A sailor, his bride and their black dog going to see some dry places that have Geocaches in a coach made the year we married.