Sounds like a ground problem for Gen start circuit.
The Gen starts from your chassis batteries and they start your engine so boost will not change anything for Gen if you use it.
You need to chase the ground leads for GEN to the chassis frame and may need the battery lead cleaned and tightened or the ground leads off start switch on dash maybe loose.
Not sure it will help you but there is a
dash wiring diagram on page 20.
All the links in my signature below have links to many items for your Newmar.