Coconut Oil before use, and simple green concentrate overnight before dumping has always worked for me. The coconut oil keeps the tank level sending units and valves lubricated as well. The folks that we bought our first camper from told us about these tricks, and the tanks never smelled or clogged up in 18 years.
2013 Palomino SolAire 317 BHSK
Nights Camped YTD: 39
TV: 13 F-150 Screw EB Max Tow, ProPride Hitch
1995 Jayco Eagle 304 BH sold- March 2013
1978 Nomad 18 sold- April 2005