We have used the TP, from Costo, since before we had the RV, we had a VW van, that I equiped for camping, and behind the driver's seat I had a porta poty, for the rainy day, that we did not want to get wet.
We are still using TP from Costco, and the liquid that I have been using latelly is purchased at Home Despot, made by Zep manufacturing, blue in color used for ceptic tanks, so far so good.
I have not been able to procure calgon to try, down my neck of the woods nobody seems to know what i'm talking about.
Try to meashure your black tank in the bottom area, and then calculate how much water you need to place about a 1/4 inch of water after you dump, so that there is always moisture on the tank, when you are using it.
Any single ply TP will work. I have already tried the cup test. Put a single sheet in the glass of water, cover with hand and shake the glass. It breaks down right away.
The problem I've encountered with single ply TP is that it's so thin, I have to double up with it to keep my fingers from getting... well, you know. So, I typically use twice as much to do the same job as double ply TP does. For the life of me, I can't finger, excuse me, I mean figure out the advantage to using single ply.
We also just use Costco's Kirkland 2-ply tissue and it works just fine and I even keep my fingers clean.
Ron & Sandie
2013 Tiffin Phaeton 42LH Cummins ILS 400hp
Toad: 2011 GMC Terrain SLT2
Tow Bar: Sterling AT
Toad Brakes: Unified U.S. Gear
TPMS: Pressure Pro
Member of: GS, FMCA, Allegro