Another "scrubbing" remedy...
3-4 bags of ice before leaving the campground for the next...the ice will "scrub" the walls and floors of the tank and as it melts, will help to keep it suspended (sloshing around) until you dump it at the next site.
As for "dissolvers" or stuff to add; most are ineffective, some are even banned from a lot of campgrounds. These digesters require time and lots of it, not normally found in RV tank usage and they are actually counter productive to the natural enzymes found in the black tank. The only "process" that seems to have good results is the "Geo method". I've never used it; plenty of water in the tank, dump, fill the tank about half full again and dump, then let the rinser run for a while. Your black tank is a temporary storage tank and shouldn't be confused with the holding tank of a septic system where you months for a digester to do its thing.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)