The only thing we use in our black and gray tanks is Costco brand liquid Laundry detergent drop in pillows. I put 4 of the pillows in each 50 gallon tank dissolving them in the toilet and bathroom sink first. I normally brush the toilet detergent before dumping into the black tank. Never have any oder even in hot weather. We normally camp for 4 to 5 day without dumping either tank. I always keep both tank valves closed even when we have full hookups which isn't very often. I only dump my gray take when it almost full, the full tank dump helps clean out any food waste that gets in the tank.
Chuck & Teri, 1999 35 ft. Dolphin 5350, F53 Chassis with tag axle, Chassis build date 1/99 in Mexico, Banks System, Trans Command, Air Lift Air Bags on rear, Koni Shocks, Blue OX TruCenter, TigerTrak track bars front and rear, Roadmaster 1-3/4" rear auxiliary sway bar, 2013 VW Passat TDI SEL Premium, 1994 36 ft. Avion 5th Wheel. FMCA #F430129