Take a very good flashlights.. and look at armature slip rings.. they need to be nice and shiny.. don't believe me ? You tube FLIGHT SYSTEMS GEN REPAIR.. or onan gen repair.. flight system gen repair are ... our go to guys.. if you don't clean them.. the circuit board will shut gen down if not enough voltage.. so nothing to take apart.. just look where the brushes are and look inside at slip rings..
Now you are not giving us codes.. so it is the 1st thing I would try, after fuel filter change.. unless you look and see they are dirty.. then if dirty.. get a slick stick from flight system.. and clean.. following you tube instructions.. takes 5mins.. usually nothing to take apart.. and I got mine under 50. But you can make your own.. but at your own risk..
You tube slick stick.. maybe that will work..
As always good luck and let us know what you did and maybe better members then me will help you
2000 southwind storm, workhorse custom chassis with 7.4l vortec