We have a 2000 Hymer 644 C, the dealer we bought it from told us the exterior joints should be resealed every couple of years. There have been a couple of small leaks, but those have been fixed before they caused any damage. The camper is nice and dry and all the joints seem to be original and look tight. I don't really see how it could easily be resealed other than running some caulking along the edges, without taking the whole joint apart. So I'm thinking that if it's not broken don't fix it. But I wonder what the general practice is? I don't really trust what the dealer told us.