Okay, if it is a 6 GEA that would indicate that it has an electric option in addition to operating on propane. Can you confirm? Do you have one or two switches on your panel for the water heater?
Assuming the above, 4 different systems are needed for everything to go a planned: 1) 12 volt DC (battery, like in a car) power is needed to operate the WH brain, spark igniter, etc. 2) 120 volt AC (generator/shore) power like in a house to power the electric heating element, 3) propane to fuel the gas burner to primarily heat the water in the water heater tank, and 4) fresh water supply
If the water heater is not working AT ALL, then need to troubleshoot all 4. Let's go backwards. Water, you were hooked up to city water and had ample cold water to faucets. Are any bypass valves or anything turned incorrectly that would prevent water from filling the HW tank?
Propane- If the range burners and other devices worked, then you are probably getting propane to the WH too. One thing newbies can do wrong is when turning on the propane cylinder valves, it is done too quickly. For the first half turn or so, it needs to be turned about as slowly as possible until it is apparent that the lines are charged. Too fast and the safety valve thinks there is a bad leak and cut off most of the flow. Once the lines are charged you can open the valves up the rest of the way as fast as you want.
120v power- Properly plug into shore power or generator. Confirm 120 power to RV by checking power to plug-in outlets. There should be a flip breaker in the RV electrical panel for the WH. Move it to the OFF position, then to the ON position to properly reset it.
12 volt power- Confirm on by testing other 12v stuff like lights or water pump. Check/replace 12 volt automotive type fuse in the RV's electrical panel that goes to the WH.
That's all standard DIY stuff to check. Test water heater operation. Turn on the propane switch. A light will often stay lit if the WH did not light and there is an issue. If this happens, try turning the switch OFF for 30 seconds, try again. If light still stays on and WH doesn't light after trying this a few times, then further diagnosis is needed.
There are a couple of things specific to your model that can be checked, but it may not be safe or good to try as a DIYer if you are not very handy or familiar with electrical. It may be best to hire a mobile RV tech or take the RV into a repair center. 1) there is an additional 12v fuse at the WH inside of the panel accessed from the outside of the RV. It's a smaller mini ATO type fuse. If that is blown, it would prevent operation. 2) It seems like a very common issue on these models is a blown thermal fuse. These items can be viewed by searching YouTube.
Best of luck.
