Considering purchasing a 2003 Monaco Diplomat 40 PWD

I can't help you with the specifics of the Monaco. Double check he source of the leak. If it was from the roof you could have mold and extensive damage in the walls.

Is there any evidence of roof leaks, stains or soft spots? If you are not familiar with RV's consider hiring a mobile RV Tech to do an inspection BEFORE you give the seller any money or sign any agreements. It will be money well spent.