Black water tank won't dump

[jonthebru]

I'm a nobody here but I have encountered many sewage clogs through the years. I think the worst problem she has is the lack of a water hose with pressure. Somehow water needs to be delivered to the tank either from the top or through the outlet at a vigorous pressure. And in reading the comments for the HD product it may need several efforts to succeed. I would consider starting at the bottom with that hose feed device attached to the drain hose.
So get someone with a tank of water on their truck and a pump maybe someone who washes cars on site or sprays farm chemicals (use water only of course.) Or a friendly fire truck! Once the yuck! is broken up it should clear itself.
It may take some money and time... Good luck
I know I am talking to the doorman out front, she is travelling at the moment, but what the heck.

 

[Zil]

I am not surprised that it is a mixture of caustic sodas. I want to be sure the OP is not surprised by the reaction. Based on the information posted by OP, the tank is not solid packed but clogged at the dump valve. But then that is only my opinion, and worth as much as OP paid for it. But I don't recommend harsh chemicals in the quantity needed for the holding tank, unless the tank will drain on it's own, at least slowly. That info is also worth what you paid.