I don't know, maybe I'm making it too easy. As a river runner for the past 27 years, and having had to deal with numerous loaded porta potties after river trips, all I can say is the trick to a positive and effective dumping experience is liquid... the more the better. Keep the word "SLURRY" in the front of your mind. Attaining a slurry is your objective in managing the black tank!
As others have suggested, add a couple of gallons of water before leaving, along with a good treatment (I use Odorlos, the stuff is awesome). You want to avoid the creation of any solid "fixture" in the tank. I also keep a 1-gallon jug of water by the toilet, so that a lot of water can be added to the toilet to ensure everything flushes, and more water goes into the tank. This will completely avoid creation of a solid "pillar" in your black tank.
Once the tank gets full enough you think it needs to be dumped, top it off with more water, drive the trailer around to mix everything up, then do the dump at your convenience. If you've been generous with adding water during your stay, this step may not be necessary.
Adequate liquid ensures everything will exit during the dumping process. I have never felt the need to flush it out with a wand or hose following this routine. Although admittedly I have also never stuck my head or a flashlight into the toilet to check, I am fairly certain there is no build-up of anything. The black tank sensor has always worked, and I've never detected any odors. This is just a super easy situation to manage, in my opinion. Much easier than a river toilet, that's for sure!
^^^ This ^^^