Lots of information on this site if you search. Obviously you are looking for signs of leaks. GEt on the roof, look at the seals and caulk. Press around the windows outside and inside looking for signs of leaks or soft wood. Step everywhere on the floor looking for soft spots. Check all appliances, Check in each cupboard for signs of leaks. check tires, etc. Bring someone knowledgeable with you. Jayco makes a great unit and is reputable. Make sure the slides work easily. Search on google for what to look for used rv, there is a great youtube video on avoiding a lemon.