Truma tells us that this should be done “occasionally” and:
The appropriate product to use is vinegar essence or citric acid.
Put it in the fresh water tank, the mixture being : 1/3 product to 2/3 water.
Have it go through the water system, allow it to work over night and then rinse the system with fresh water.
(EDITORIAL COMMENT – The Truma water tank is only about 2.5 gallons so you don’t need to make a mixture to fill your entire water tank. Empty the tank and just make about 5 gallons and that should be enough – maybe even less.)