When the house batteries are low, merely starting the engine will not suffice to start the generator. if your motorhome is equipped with an auxiliary switch, you have to press on the on position in order to connect the engine batteries with the house batteries. As far as priming, you should only need to do that if it has been sitting a couple of days, and then only for about 6 seconds. If it does not start right away, then you most likely have another problem. Use a volt meter to measure voltage at house batteries. Make sure all connections are tight, and that the disconnect switch is in the on position.