As far as what to look for, check in every cabinet, nook and cranny for signs of moisture. Use your moisture meter on any wood you can see behind covers etc. and take a thorough look at the roof. Look for soft spots, cracked seams or caulking. If you smell mold it is not a good sign.
Look at the frame underneath to see if it has a lot of rust or if some of the frame has cracked and take a good look at the plumbing for signs of leaking or homeowner "repairs".
Water is your biggest enemy in an RV. Look at the repair/remodel section to see where a little leak goes. Good luck and happy hunting.