Here is a testimonial from Warren Leith regarding the necessity of cleaning your roof a/c units thoroughly:

The condensate drain to roof plugged on one unit and condensate came inside of coach. I went up on roof and unplugged it. It's happened before. He asked me if I'd ever completely dismounted and spray cleaned the whole units coils in and out. I hadn't! I've brushed them off but did not spray clean them. He took the units completely apart from inside and dismounted units from roof and used an acid cleaner on the coils. They were black inside and out. They are now silver color again. When he washed them off with his sprayer, a stream of black water about a foot wide ran down the side of our coach for at leaat a couple of minutes from each unit. It was very graphic & looked like coal slurry. Needless to say, they move more air now & cool better. He told me most people aren't so lucky with their AC units - they get plugged up and then the motor burns up and the entire unit has to be replaced.