I do not use Calgon, but I do use Arm and Hammer Washing Soda to soften the water. In my case it is partly due to my really flat tanks which are part of the extended season package. The washing soda softens the water and it for sure makes the waste come out easier.
I use it with Happy Camper and have not had an issue. I also have used it with Sea Zyme, (bacteria and enzymes) similar to Happy Camper. I have the poweder and I do have to mix it up in a little plastic jug with a lid, shake well with water, warm work better and then pour down the toilet. With the powder if I do not premix, it gets hard and crusty in a glob.
As Gene said, the soft water makes things slide out easier. I do not know if it will do much for bio breakdown action if you have built up "stuff". The extreme clean may dod a better job of that.
Since I clean my tanks well after every trip, and have internal sprayers in both black and gray, I have not had any build up. I am fortunate that days when the dump line is long, I dump and run... to not hold up others in the line. Then once home and unpacking I can put the sewer hose into my aerator tank septic system and finish cleaning up with the sprayers. I also at the end of every campout, transfer all the left over fresh water into the black thank so it is usually always full of liquid before I dump. I have never tried it any other way, but dumping a 1/4 full tank with not much water inside I'm sure will leave a lot of paper and solids behind.
My T2499 had a large slope towards the drain fitting and it drained out well, but this T310 with the flat enclosed tank not so easy.
Hope this helps and good luck
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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