at the end of the camping season or between trips that are weeks or months apart they make some wands that fit down through the toilet opening so you can move it around to clean the residue off the sides of the tank. You can get some soap dispensing containers that screw onto a garden hose if you want to use a cleaning agent instead of just plain water for washing off the sides of the tank.
The containers that screw onto the garden hose are sold
at garden supply outlets. They get used for dispensing fertilizers, pesticides, etc. They are calibrated and can be set with a dial to release a particular number of ounces of the solution per gallon of water that passes through the hose.