Honestly, to date I use that gel stuff that you stick to the side of the toilet bowl so some goes down with every flush. I've noted, though, that after a few flushes it seems to dissolve down into the tank.
After emptying the tank, I use the back-flush until the water runs clear (as the manual advises). But I haven't considered whether to leave anything in the tank.
I do leave a little water in the bowl as an extra safety from bad smells going into the trailer, but so far haven't had any problems.
Note that I'm a newb, just got my trailer last fall. If I should do this differently, I'm all ears... interested in the experience of others.
2012 RAM 2500 Megacab Laramie Diesel
2016 Jayco Eagle 284BHBE
ProPride 3P WDH