I use White King D detergent with softener in it, 1 box lasts about 2-3 years. I also use an enzyme type additive in both black & grey tanks. Woks great for us. When we leave for home, I flush tanks thoroughly and add about 1/2 cup of baking soda in each tank and put several gallons of water in each to let it all slosh around while driving. So far, so good.
2004.5 GMC SLT CCLB, Ride Rites, Turbo brake, EFI Live
2012 Arctic Fox 29 5T 5th Airborne, B&W, Prodigy
Papillon navigators Lily, Buddy and Willow