Great to hear about the water. The spitting is most likely air, as you said. Running the water will bleed it out.
The hot water heater will have a outside door on it could be on either side of the RV.
The door will swing down, it has a spring loaded thumb latch at top center of door. It also has a small aluminum vented screen in the door.
If you have water already coming out of HW faucets, then either the HWH has a bypass set, or the tank is already filled. To check to see if it's filled turn on the water pump(or have city water connected) and open the blow off valve and see if water comes out. No water means the tank has a bypass. If you have water then HWH can be started safely.If no water is present, than do not try to light water heater until it's filled.
If no water then, go inside of the RV and find the back of the hot water heater. Most likely inside a storage cabinet, or a snap fit access panel.
Once you access the back of HWH you should see some valves and piping. If it is equipped with a bypass, you should see three valves turn the crossover valve so it sits cross wise to the pipe. Turn the other 2, so they are inline with the piping. This will allow the HWH to fill. Turn on the hot water tank and allow to let the air out of the system by running the water..
Happy to help we all started at the same place. just helping it forward