You probably have done some/all of these things, but here is a list.
Keep valve closed and only dump every few days. At that time drain it and use the back flush. I prefer to run it with the valve closed for 3-4 minutes(that doesn't over fill it) and then dump to get it clean. I have a extension thing on the drain that is clear so I can tell when it is clean.
Then close the black valve and drain the gray tanks. I don't stay in one place long enough, but those that do recommend leaving the gray valves open until a day before draining the black (so you can have some water to clean out the residue from the black tank).
And make sure you don't skimp on water when using the bathroom.
Tom and Gail
2013 Mountaineer 362
2012 Silverado 2500