I tried a version of the GEO method last time out. 1/2 cup dishwashing liquid, 2 cups Borax softener, and abt 3.5 gal of water. I dumped the mixture in the tank before leaving the house. After 3 days camping absolutely zero smell of any kind. Even the blue stuff I had been using starts smelling after 2-3 days, not bad just a strong chemical smell. With that mixture, no smell. I dumped and flushed before leaving the campground and when I got home added another dose of the mixture ready for the next trip. No smell even after 1 1/2 months sitting... Yeah I know we haven't had much camping time lately.
Clif and Millie
Ozzie (half a dog tall & 2 dogs long)
2009 Ford Lariat F350 SRW CC Diesel
2013 Cruiser CF305SK Patriot Edition