Seafoam makes stabilizers to use if the fuel is going to sit in the tank for a long period of time. If you have a pre-2008 engine then a lubricity enhancer is not a bad additive to put in the tank. No need for a cetane booster unless you want to drag race or do sled pulls with your RV.
As for water it is important to maintain the fuel filter and water separation unit and drain it periodically. Different for each engine so learn where yours are located and read the manual as to the maintenance required. And avoid biodiesel if at all possible and never put in B20.