If you ask me, seems like the best idea in the world would be to make your own pressure testing rig (that can blow into a window) from an electric blower and use it to test the roof of any RV you are thinking of buying.
I have documented my experience elsewhere. Visible water damage in the cabover equaled a complete teardown and rebuild. I'm talking hundreds of man hours all by myself and many more with help when assembling larger sub-frame structures. If you are not ready for something that can result in that level of work, then do not buy something with visible water damage.
Jose