Been my experience that almost every used unit has at least some intrusion issues (and I've looked at dozens of them). Even found damage on new units on dealers lots because, once the unit is built, it goes outside, in the elements and stays there, from the builders yard to the dealer, always outside and today, builders don't take the care in sealing a unit properly and end users don't maintain seals so, they rot.
RV buyers are under the false assumption that they can let a unit sit outside, in the weather, for an extended period of time with no pro-active care and it will be fine. Don't work that way.
Without continuous seal maintenance and inspection, every unit will leak and rot, even an aluminum framed unit because the interior walls are still wood or some wood based substrate.
Basically, it distills down to 'how much rot' is on a used unit and if you want to remediate it. Some units are past fixing, some can be fixed, all distills down to what you want to do, camp or work on a unit.
I read a ton of posts on this site about rusty screws and delam and stains on interior walls. All of those issues stem from water intrusion.
have fun and decide but whatever you decide, take continuous and pro-active care of the unit.
2015 Backpack SS1500
1997 Ford 7.3 OBS 4x4 CC LB