We have always use these (we call them swamp coolers) on our house here in the desert and yes, they are only good with very low humidity. Typically, the water pans are leak proof, but if I had one on my rv, I would definitely make some sort of drain that I could access easily for when we would travel. A simple 1/4 or 1/2 horse AC motor is somewhat efficient, but you also need a small water pump.
Another con is that they are not small and would add sufficient height to the top of the rv and would present a large wind drag, so securing it to the top could be difficult.
The worse con is.... boy these things are really, really ugly.
Ron