Another possible problem could be your propane tank safety valve.
Propane 101 - Promoting Propane Safety Through Understanding
New tank connection systems are equipped with a shutoff to prevent a catastrophic release of propane gas in an accident where the rubber line is severed (called a free flow shutoff). This emergency shutoff does not completely shut off the gas but allows a very low pressure to bypass so the lines can be filled when you first open the valve.
If the tank is opened too quickly and the lines are empty, the tank valve may detect an emergency situation and shut off the majority of the pressure. The remaining pressure may be enough to fill the lines and run the pilot light and even a single stove burner on low flame, but not enough to run the furnace, fridge, or water heater.
1) Close all propane tank valves - make sure pigtail is TIGHT.
2) shut off or turn off all gas users inside the camper.
3) WAIT at least 5 minutes - WAIT
4) Select the FAR tank on the selector regulator
5) WE ARE GOING TO OPEN THE CLOSE
6) SLOWLY (about 30 slow count from closed to open) open the Close tank valve. This should charge the short hose up to the regulator to full pressure without tripping the shutoff.
7) SLOWLY rotate the selector to the tank that is open and observe the red indicator goes green
8) Enter the camper and turn on 1 small stove burner. (Have a match ready in case your sparker is not set up properly)
9) try to light it until it lights or you smell gas. (If you smell gas and the sparker won't ignite the burner use the match.)
10) Once you have a good burner on high with a nice blue flame open the other burners and start them. ALL burners should light and run on high. If the burners go out, the OPD valve on the tank has shut off the gas. You should observe that the red bar is again showing. Repeat the above from step one.
11) Assuming the burners are running, you can shut them off and start up the water heater and see if the problem you are having repeats or is solved.
If this fixes your water heater issue, then SLOWLY open the FAR bottle and rotate the selector to the newly open tank. Go back inside and light one burner. It should light immediately and "blip" as the air in the line leans out the propane; then return strong. Go outside and examine the regulator and it should show green no matter which tank is selected.