@tomseeley,
We had a similar problem, but it only happened when we were a little off level on one side of the RV. Ours turned out to be the Seal was Old and needed to be replaced. I'd take off the inside cover and take a look see.
Our drains were NOT clogged, but the unlevelness of the rig caused it to back up for some reason.
As the others have said, be sure to clean the coins and straighten any of the blades out while you are looking at it all.
BTW: If it's the seal that needs to be replaced, I would recommend a DIY! It's easy to do!
Livin' the Dream, Over the Road