How rotten is the wood? Can you easily stick a screwdriver into it? If it is soft, but not falling apart, I'd try to go the GitRot (thin resin) route. You can drill 1/8" holes 3/4 of the way through, repeatedly, to honey comb it, then pour on the resin and let is soak in and cure. The wood needs to be dry first.
I agree with Tom. It looks to me that it can be fixed by using GitRot. Boat builders use it all the time to keep boats from sinking.... I don't think you camper will be exposed to as much water as a boat!