I had same problem with my onan 4000. It was in rv shop for 3 months for warranty work and the carb was performing the same after sitting. Tried carb cleaner no go. Removed carb and took apart carb bowl and solenoid. Both were coated with a sticky resin/varnish from the gas. Used sea foam and carb cleaner to clean both, reassembled and added some sea foam to gas tank also. Also replaced the fuel filter and air filter and spark plug while I had it apart. Runs like a champ 2 yrs later. Run it under load for an hr every 6 weeks and add stabilizer to tank prior to storage to prevent reoccurrence.
2018 Intent 31P
Safe-T-Plus, SumoSprings (front & rear), Snappads