Hi and welcome to the forum. I don't know your coach specifics, but since no one else has responded so far....... It sounds like (from your description) that you have a tank style water heater. So propane is on, as you say the stove lights OK.....and I assume your furnace also operates OK?
The light on the water heater switch illuminates and cycles with ignition attempts, so likely you have 12vdc to water heater control board I would "expect". When you turn on the water heater switch can a second person outside near the water heater hear the gas valve click open and then spark trying to ignite the gas? Does the gas actually ignite (flames) OK, but then go out shortly after? If you get to flame OK, but then it goes out and tries to restart again, it might be a dirty flame sensor.
Your water heater manual should suggest other reasons for failure to ignite. If you are not familiar with working on gas appliances though, I would suggest hiring a mobile tech or have an RV Dealer help you resolve this. At least though you can advise them with a little more information about where you get to (or not) in the ignition process.