HI,I usually replace the sacrificial rod when its 80% gone. Its a rod of sacrificial material about 12" long for my RV water heater. It loses material by combining with the minerals in hard water. When it has been eaten away about 80% it unscrews from the bottom of the tank and a new one is installed. I flush the tank by removing the rod at the bottom of the tank and the pressure relief from the top and run water through until it runs clear. I then reinstall the relief valve and rod and let the tank fill before turning on the propane. Living in a RV you become your own electrician and plumber.Here is a link to a photo and description of the use of the rod. Ralph

http://www.waterheaterrescue.com/pages/W...