Water heater replacement is not too difficult in general. You first have to disconnect the hot and cold water lines, and that assumes you can get to the back side of the heater (from the inside of the coach). If everything is original, you will be dealing with copper lines and flare connections most likely. Then from the outside you remove the propane line, another flare connection. The water heater you will get is the same unit in size and everything else other than the propane connection which will be in a different location. You will need to decide if you wish to use the new location and re route and replace the original propane line, or if you want to try to drill a hole in the new unit, where the old one came in. You will also have to decide if you wish to remain with a pilot light type, or switch to a new electronic ignition type (I vote for electronic ignition).
Then you will unscrew all the screws from the outside flange, and inserting a putty knife cut the original sealing material all around. That will be your hardest job, and it is helpful if someone is inside pushing the heater out, while you cut the sealant free.
Then it is a matter of just putting the new one in place with new sealant tape and screws, hooking up the propane line (see above) and the water lines from inside. If you go with electronic ignition, you will have some wiring to do from the heater to the switch, and you will need a source of 12 volt
power for the supply.
Lining up the pipes inside can be a challenge. You might want to change the hookup to something easier, such as PEX if you run into problems.