I purchased a used (2003) 5th Wheel Camper and have discovered that the “Black Water” holding tank isn’t completely draining due to a build up of toilet paper and etc. I suspect that the drain was open and the fluids drained off leaving the rest. It seems to be flushing better each time we dump the tank but I can not see into the tank to verify this. As a result of the suspected residue on the sides of the tank, the tank level guages are not accurate. Is there a chemical a person can put into the holding tank that will disolve any paper or poop that is hanging on the sides or bottom of the tank? I spoke with a plumber and he suggested “Thrift” a crystalized product and then fill with hot water. The tank and connections are plastic/rubber. Any help would be appreciated. Thanks.