Sorry but I think half of the people that repair RV's just take advantage of usually older people who can't or don't know how to fix their stuff.
$4,000 is a complete rip off. New Onan's as stated are $3500, and that often includes installation. But usually it's just a simple fix on generators. An honest tech could pin point the problem and most likely fix it for much cheaper.
The dishonesty out there is getting really bad and taking a lot of fun out of owning an RV. When people are confortable charging $4000 for a generator repair, we are in real trouble.