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Bobbie, if there is a solid “mound” in the tank it might require a professional spray/steam flush out with moderate pressure. Depends how solid the mound is. Some folks empty a few bags of ice into the tank and drive around for several miles so the ice can break apart built up solids.
As for Happy Camper I have been using the product for over 10 years and I am very satisfied with it. I have had no problems, no odor, easy to use. I use Happy Camper in both holding tanks and dump at the full mark. Of course you don’t want to drive around with that much waste in the tanks but if you are parked for a while it gives the “bugs” a chance to do their work all around the inside of the tanks.
A friend of mine had the insides of his tanks “pressure washed” a while back and is satisfied with the results.
Happy Camper comes in several sizes of tub type containers. Here is a link to the Happy Camper product: https://www.5starhappycamper.com
On the right bottom of their Home Page you will see a new product that might take care of your problem.
Subject: holding tank buildup
You have a new contact request
Name : Bobbie
Subject : holding tank buildup
Message Body : I would like more info on the Happy Camper product for our black holding tank. Have been told there is a “mound” in it and would like to break it up and flush.Thank You