Don't waste your time looking for "the infamous little black vent tube". That's _only_ on the 2500LP units, not the gasoline 2800. It's a breather tube for the propane regulator.
At 50 hours and 10+ years, your issue is almost sure (95%?) to be a gummed-up carb. Onan says they're not rebuildable; I believe them. They have super-fine internal passages. I have cleaned two of my previous ones, but they weren't that neglected.
Assuming it can be started and run at all, and using all proper caution, break the fuel line somewhere between Onan & fuel tank. Put the 'to Onan' end into a can of 1/3 Seafoam, 2/3 fresh gas. Start it, let it run for 1/2 hour. Stop it, let it sit overnight. Run it 5-10 minutes the next day to draw in fresh solvents. Repeat daily until it runs right or you give up. I succeeded after about a week in one case.
Jim
