[DQDick]

1st you are extraordinarily lucky your gages work. Many of us have had non-working black tank indicators since day 1. The only thing that I can think of that would cause what you are describing beyond a blockage of some sort (which you seem to have checked for) is that you have a leveling issue with your tanks. Either it's not down hill from your tanks to the outlet or the dump station isn't level to the point where things would flow downhill correctly which could explain why it works right on one side and not the other. Is this happening with both the Grey and the Black tank or mostly just the Black? If it was black tank only then it could be the pyramid of poop situation building up from time to time.