Charmin... just watch out for the bears
Seriously you want something that will break down easily. As a general rule the single ply RV stuff doesn't seem to do as well. We use Charmin or Northern along with plenty of water and have never had an issue. I also subscribe to the Geo-Method
of keeping the tank clean. My standard method is to put about 5 gallons of hot water with the softener, bleach and detergent in the black tank before we head out, then empty the tanks at home when we get back, this keeps things stirred up, when emptying them, I make sure they are very close to full then rinse several times.