I think that what people refer to as 'dry rot' is actually water damage. The particle board, waferboard, or even regular plywood, will turn to mush behind a wall covering if it's exposed to water for a while in that closed space. Look for soft spots on otherwise sturdy floors and walls, check bent trim pieces carefully. Discolored ceiling near corners or junctions with telltale stains show that there was water entering at one time, you just won't know when it happened, or if it got fixed, unless it's still wet.
The point where the seams meet on the roof, the vents, air conditioner gasket, skylight, all can be cheaply sealed with one or 2 tubes of Dicor self leveling sealer. Use non self leveling if you see a cracked seam that needs to be filled and isn't very level (it's just thicker so it won't run). Don't use silicone caulk, it leaves a residue that ruins future bonding.
Don't leave the windows open. Add a vent cover before you leave the overhead vents open, to keep out rain. With a dehumidifier, carpentry skills, and some patience, you can safely fix damage that many people would give up on. I carry some Eternabond tape that is like very sticky caulk tape, for fixing tears on the roof or leaks pretty much anywhere on the roof, plus a tube of Dicor with a cheap caulking gun.
If you do spring a leak and happen to know about it right away, quickly stop the leak and get lots of air to the wet parts. In my old rig, when I had a burst pipe flood the whole floor, I put the dehumidifier in the shower/tub with no pan to empty, and cheated it so it would run nonstop, dripping the water down the drain. Not a bit of mold, since it was stopped as soon as it happened, and dried quickly and totally. In other buildings, when I've had a flooded room, I just pull the floor covering up quickly, remove the water, and spend lots of time with a dehumidifier.
Once you tear into something that needs fixing, you will see a manageable level of complexity, and you might enjoy the process of keeping that thing alive and kicking. I think there is no better bang for the buck than an older class A.
