Sudsy's coach may work as he described, but my present DP Tour Master and other coaches I am familiar with use the cross connect of the house and chassis batteries to start the main engine directly. On my T40-C the momentary rocker dash switch is labeled Aux Start and is on the left end of the top row of dash switches.
At least on my coach, the generator starts from a third set of batteries that also power the inverter for 110 volt appliances in my rig like the residential refrigerator. That stack of 4 each 6 volt golf cart batteries (in series/parallel configuration) make up the inverter source with over 450 amp-hours of capacity.
2007 Tour Master T40C
Acura MDX toad
"It takes a great deal of time to recover from any improvement..."