This was fun. For Horizon/Journey, here's the drill Drain the water heater and turn off the water pump. Move MH to a dry spot, and lift rear on jacks. Turn off propane and diconnect the gas line at the burner. Remove all the screws around the outside perimeter of the HWH. Slide under the waterheater compartment and remove the rectangular plate on the bottom of the compartment, and the 6" marine inspection port on the back of the compartment.
Go back to the front and with a putty knife or utility knife, go all the way around the perimeter cutting loose the caulk behind the mounting flange. You may need to remove a blockoff panel on the side of the HWH nearest the rear of the MH. Tug and pull on the waterheater until all caulk is broken loose and slide it out about 3 or 4 inches. Push the gas line back through the hole with the rubber grommet as the HWH slides out.
Then, from under the water heater compartment and working through the round hole and the bottom access opening reach up behind the HWH and unscrew the PEX hose on the brass checkvalve. Small water pump pliers are great for this. Then find a socket and ratchet that fit the hex on the checkvalve and unscrew it.
When you get it out, knock all the ol nasty stuck parts out with a suitable punch, peel out the rubber seat with needlenose pliers, put new teflon tape on the threads and begin reassembly. While you are in the neighborhood, go ahead and do the same thing to the cold water inlet checkvalve on the lower port...unless you are really likeing this enough to want to revisit this area in the future. Figure on about two hours one Sunday morning.
See wasn't that fun?