Leaks..as mentioned are the killer to be super watchful of. It's not the water coming in, it's the rot that is the result of the water coming. Do a Google search on wood rot and it's health issues. Be aware too, that fixing an RV that has the wood rot is a huge HUGE expense. One you do not want. IF you are handy and can repair yourself? That is a whole different story.
One answer...maybe the answer, is to do your research on RV's that are built using aluminum framing. Don't get trapped buying an RV that is water damaged. Everything, anything, else can be managed. Not water damage/rot.
Gary Haupt