First Welcome to the forum.
Many times your questions can be answered by using a few key words in the search box at the top of the page. In your case a search on "water heater" would have returned several hundred similar questions. Also posting a new problem as a reply to a "solved" thread will almost guarantee no one will look at your post. I moved it to a new thread and named it something that should get the attention of most members who are dying to help you.
The original thread you posted to Water heater not working on AC (SOLVED)
had the answer to your question (pretty much) as it contained the wiring diagrams for your heater; both the AC side and the DC/Propane side.
Note: I mention DC/Propane since both are required to fire the water heater on propane. You also failed to mention your model of water heater or even the year of your camper. This is important because some older water heaters had manual pilot lights. Since you say you have a "fault/reset" light I assume you have a direct spark ignition like the previous thread's wiring circuit.
To run on AC all you need to do is flip that switch (photo in previous thread) to ON and have a good element (not fried by turning it on without water in it).
To run on DC/Propane you will need a good battery, fuses, and, well, propane.
When you first switch on the propane (in the camper switch) the "fault/reset light comes on when the heater is calling for "fire." The Fault/Reset light will stay on until the water reaches the correct temperature determined by the high limit switch. Then it will go out.
If it stays lit, it is because "fire" never happened.
It will try to light 3 times and each try can take over 5 minutes for a total of about 15 minutes. If you do not get a "light" after the control board tries three times, the system will shut down and will not work until power is removed from the water heater by turning off the switch; waiting 3 minutes; and turning it back on again. This is to prevent a buildup of propane in the cabinet that could ignite in a spectacular way.