Bleach is not good for the black tank. Simply use enough water. If you need to clean it, add a small amount of Dawn liquid and/or Calgon and lots of water. You should not have any issues with either tank if you follow some simple rules....
Never leave your black tank valve open.
Use plenty of water when you flush.
Empty black tank when it's about 3/4ths full.
Flush black tank after emptying.
Leave a couple gallons of water in black tank at all times.
Use single ply tissue.
Using these few simple rules you'll never have a problem with your tank. Bleach will damage the toilet seals as well as the knife valve seals. "Saving" used toilet tissue is unsanitary and gross....just sayin.....Dennis