Onan Propane Genset issue - won't stay running

Background: Gen has been excercised every month or two for about an hour with the AC on/off (loaded).

I let it sit for two months this time. Propane is on and functioning in stove, fridge, and LP vent (BBQ)


Problem:
Generator will start as normal, but when I release the switch, it shuts off.

I've tried unplugging/plugging shore power, but the same thing happens.

It will run as long as I'm holding the switch to "start".

Any ideas?

(vent tube has no mud-daubers...)