Jenny,
The only safe way to prime your water system is BEFORE you connect the mains supply.
When you arrive on site, first thing to do after unhitching and putting the legs down is to get a full aquaroll of water, plug the pump in and use the 12v battery supply only to fill the water system.
When filling, first make sure the drain tap by the water heater is closed.
Then as the pump runs, open a Hot tap until water runs without air as well, repeat for the other hot taps and shower, then do the same for cold.
Only when you are certain no more air is coming through, then connect the mains supply and switch the water heater on, or use the gas to heat the water.
If you do not follow this procedure you risk burning the electric element out.
The hot water system will NOT fill properly unless you bleed all the air out first.