Where does water damage come from? Water, obviously, but the source may not be so obvious. Water from outside the rig is often the most damaging; but water from inside, leaky plumbing for example, can cause problems, too. When inspecting an RV, do the "top to bottom" search method.
When you step inside the RV, first use your nose. You shouldn't smell mold or mildew. If you do, there are problems afoot. Many folks will simply turn on their heel and walk away from something that smells.
Lower your sights, this time check around all windows and anything else (range hood vent) that makes a hole in the walls of the rig. Many RVers aren't aware of it, but windows need periodic maintenance to keep water from seeping into the rig from between the window and the rig sidewall. Warped wallpaper or discoloration is again a sign that there could be a problem with water infiltration.
Finally, get down low. Peek inside lower cabinets in the kitchen and bathroom. Open spots where equipment is located. Look for any sign that water has been where it should not have been. Step on the floor wherever possible--it should be firm with no "give" and no soft spots. Leaky plumbing can drip onto floor level and run downhill.
What if you find evidence of water leakage? If you really "love" the rig and don't think you'll find anything else like it, then you've got a decision to make. It's probably well worth the investment of having a qualified RV repair technician take a look at the rig. Water damage not only looks bad, but can compromise the integrity of the rig, rendering it useless for use, or costing seriously large amounts of money to repair. Mold and mildew inside a body cavity can send out spores that can create health issues. Buying a rig that has water damage is a gamble, at best.