What is different between the wiring for the interior start/stop switch and the one on the generator? The switches are in parallel, the cable to and from the battery and the output of the generator and the connectors seem to be good. So what is in the circuit from the battery to the interior start/stop switch. I don't know, I'm a motorized guy and if it was my rig I would say it was either the battery control solenoid or the battery disconnect solenoid. My guess would be that you have a very high resistance set of contacts and the load from hitting the start switch is opening the contacts.
Like I said, I'm not a trailer guy so I'm not sure of how the batteries and the truck is wired, if it were me I'd look for a wiring schematic or hopefully one of the guys know what to look for.
Another thought is that there is a constant duty solenoid and the stress of starting causes that solenoid to de-energize and therefore, you lose all power. All I see is that either the start stop circuit or a heavy duty solenoid circuit is faulty in some way.
If nothing else the guys at "etrailer" should be able to help or direct you to help.
"I travel not to go anywhere, but to go. I travel for travel's sake. The great affair is to go". R. L. Stevenson
David Bishop
2002 Winnebago Adventurer 32V
2009 GMC Canyon
Roadmaster 5000
BrakeBuddy Classic II