Generator Not starting unless coach engine is on

Most MH's use the house batteries to start the genet, so my guess is that the house batteries are dead or real low. The fact that when you start the MH engine, the alternator is supply enough voltage to the battery bank to allow the gen set to start. A good voltage test is called for to determine the state of charge for your battery bank and to try and recharge them, and then do a load test to see if they will hold a charge.