Not sure how well this stuff would work on an RV that just stays in one place as it seems to me that cleaning action takes place as the liquids slosh around, but I am very satisfied with Happy Campers;
http://www.1-happycamper.com/In any case, I can't see how your tank can get clean if a significant portion of the inside is not in contact with water. You might try to fill the tank to the top after adding the cleaner and letting it set for a while. I still don't think you'll get the results if you don't agitate the liquid though. But filling with water for a soak has got to at least help.