RV manufacturers normally use one of 2 brands of water heaters-Atwood or Suburban. There are a number of you tube videos on both brands.
To clean out a water heater, first turn off the water then turn off the electricity to the water heater and let it cool down. Once it is cool, open a hot water faucet or the pop off valve to remove any pressure. Then remove the drain plug which on an Atwood is usually nylon, and on a Suburban it will be metal, and let the water run out. I use a small wand attached to a hose to then flush out the water heater. For an Atwood, install the plug. for a Suburban, check the anode rod and if it is pitted or worse, then it needs to be replaced before reinstalling. I change ours out annually. When ready, fill with water, turn on the heat source, and you are ready to go. WARNING---DO NOT turn on the heat source until the water heater is full of water.
You tube has videos on this as well.
May your smiles be many and the miles be plenty.
Karen & Allen Van Zandt