We use the Geo method but also have a wand which connects to a water hose and after draining the black tank and then the grey tanks use the wand about once a month. It is inserted into the toilet after supply water in shut off and spray it around in the black tank until you get about 1/3 full then drain again. Then we do Geo and are good for another while, you don't have to do it this way but we find it keeps every thing working and clean. Also have sewer solution for draining and cleaning holding tanks. Works great but slower (more efficient though).
good luck and fun camping