From here at the RV Repair Club.
Over time the RV waste holding tank can begin to smell. This is caused by a build-up of bacteria and other waste products left in the tank after dumping. Even if you properly dump your RV waste holding tanks and use the appropriate cleaning chemicals in the tanks, it is still important to take the time to thoroughly clean and sanitize the grey and black water tanks. It is a good idea to do this every now and then, especially before putting the RV in storage.
The best way to clean and sanitize your RV waste holding tanks is to fill the tank with a bleach and water solution. To make the solution, mix one-quarter cup of bleach with one gallon of water. The total amount of bleach-water solution you will need depends upon the size of RV waste holding tanks you have. For most, eight gallons of this cleaning solution should be enough.
Completely fill the tank with the solution and leave in the RV waste holding tank for at least eight hours. It will be best if you are able to fill the holding tank with the solution before going on a long drive. This way the fluids can swish around inside the tank, breaking up any residue left behind. Doing this will clean up and sanitize any pathogens left inside your RV waste holding tank. The bleach-water solution used to clean the tank is safe to dispose of at a dump station. Completely drain the holding tanks into an approved dump site.
If you plan to put your RV in storage or do not plan to go out on the road for a long period of time, take some time to clean and sanitize the holding tanks. This will kill all of the bacteria and ensure you have a cleaner, nicer smelling holding tank when taking your RV back out on the road.