Electric fuel pump is easy cheap repair if bought at O'Riellys or AutoZone. Symptoms you describe lead that way.
Fuel Pump (electric) Airtex E11020 EXACT replacement for Onan 149-2790 or 149-2692 $60-70
This is the pump used on all the bigger Onan diesels, assuming it is probably the same for a 5.5. Check it out. They have been known to fail at 50 hours or run for 2,000. I have replaced them every where in between. BTW Cummins will hit you for about $165 for the exact same pump with a Cummins label stuck over the Airtex number!!!