I had the same problem since I bought our '06 Beaver (Monaco, out of warrantee) in '08. Finally got sick of starting the coach engine every time I wanted to use the generator. Took it to the Cummings dealer across from the Monaco plant in Coburg, OR. The solution was simple but expensive. Monaco cheeped out in the 12v cable the ran from the battery to the generator and it couldn't carry the current (hence, big voltage drop). Replaced the cable with a #0000 welding cable (probably over kill, big $$$, almost 50' long) and the problem was solved.
Before you spend a bundle on a bank of new batteries, try this to confirm your problem. Hook a good set of jumper cables from a good power source to the gen. right where the battery cables attach from the coach to the generator assembly but before the starter solenoid. Try to start the gen. In my case, the generator started instantly. YMMV
