I'd try Seafoam, and also, if you can, add a section of clear fuel line so you can look for air bubbles. The original lines are at least 17 years old, very possibly it could be cracked and allowing air into the line. Unless you can access the top of the gas tank, replacing the fuel line will require dropping the tank. It would be nice to know if it's sucking air or not.
You mentioned rebuilding the carb. Is a rebuild kit available? Onan just requires replacing, they don't offer a kit for rebuilding.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI