You can tell if the tank is full by running the hot side of a faucet. If the water comes out without spitting, the tank is full. If it’s spitting, it's not.
Put a piece of painter’s tape over the water heater switch any time you empty the tank, so you don’t accidentally turn it on.
Water that sits for more than two weeks or so can become stale, stinky, and unfit for consumption—not qualities you want the water in your RV plumbing system to have.
Your RV water heater does require some preparation for the winter season. To make these preparations, you will need a water heater bypass valve.
While some RVs already come equipped with such a valve, others do not.
Turn off the heater, let the water cool, empty the tank by removing the plug, wash the heater using a water heater cleaner, and rinse the tank out.
The aforementioned RV water heater rinser product can be used again, and everything can be rinsed out with the pressure from the water hose.
If your RV hot water heater has a gas mode (and most do), you’ll need to clean the burner tube. This is the tube that carries propane to the burner.
Last but not least, you will want to visually inspect your water heater during your walkaround before every trip.