Newmar will answer the question with a call to there companies tech help number.
The gen-set normally uses the chassis battery to start the gen-set. with a battery boost from the coach batteries.
http://www.newmarcorp.com/get-answers/
Have the last 5 of your VIN ready to give to them.
They still service every coach they've built.
2000 Newmar mountain aire 4081 DP, ISC/350 Allison 6 speed, Wrangler JL toad.