You can pressurize the hot water tank from your on board freshwater tank or from a pressurized supply. Make sure your hot water heater drain plug is tight then connect pressurized water to your camper or turn on the on board pump (make sure water is in your fresh water tank). Make sure your water heater bypass (if you have one) is in the normal (NOT bypassed) position then open any hot water faucet and wait until a steady stream of water flowes out. That should do it. As a double check briefly open the pressure relief valve on the heater to make sure water sprays out. You should periodically check the pressure relief valve anyway so this does both checks at once.