Not much of anything.
In the winter (in California, near the coast) I dump, flush and go.
In the summer, I dump, flush a little, add the non-toxic, biodegradable, no formaldehyde type black tank stuff and a bowl full of water.
Never had a problem.
(I am never going to do anything with what's in the tank except dump it, so IMHO, it really does not have to be all that "clean")
2013 Starcraft 266RKS
1999 Ford E350 V10 Chateau (6.8L, 4.11 LS)
2005 Cedar Creek 31LBHS
2001 Sprinter 303BHS
1997 Mallard 33Z
1990 Starcraft Starmaster