I don't have my trailer at home, but based on Old-Biscuit's photo you can probably change the valve with nothing more than a channellock tongue and groove plier

- Turn off the heater and let it cool.
- Turn off the water pump or water hookup.
- Open the hot and cold faucets and let any pressure out.
- Open the pressure relief valve in the water heater and let pressure out.
- Remove the water heater drain plug and drain the heater.
- Using the channellock plier remove the hot water out (top) hose from the heater.
- Using the plier again remove the check valve from the tank.
- Wrap teflon tape on the new valve and screw it into the tank.
- Attach the hot water hose to the tank.
- Close the pressure relief valve and the drain plug.
- Open hot water faucet.
- Turn on water pump or shore hookup and listen for air at the faucet.
- Once the tank fills, turn off the faucet and turn on the water heater.
Note: Many people do not know that tools like the ChannelLock plier above have a direction of use and the strength is on the solid jaw. In other words in the plier above they are set for pulling down on the handles to tighten something. If you pushed up on the handles you would be putting the force against the moveable jaw which is dependent on how tight you can squeeze the handles and more prone to slip. Crescent wrenches work the same way.