water, water and more water... with a bit of water softener (search Geo-Method).
We use Charmin Ultra. I typically dump roughly 5 gallons of water with bleach, detergent and water softener mixed up into the black tank before we head out on a trip. I usually will have to dump the tanks a couple of times in between, but try to do it when we are at a full hookup site so I can run the black tank flush. I have done it this way for years and have never had a problem.
Aaron