The previous owner says it was never wet but in reality he has no way of knowing if he hasn't opened it up. What he is doing is trying to shine some good light on a bad situation. Several longtime RV owners and members here have given you some very, very good advice for very reasonable fee. Take it. If you are still on the fence, have a look at your future if you insist on buying that rig...
https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/29356352.cfm
Ron Dittmers post on page two of this thread that will(or should) send chills up your spine.