Disconnect the fuel line at the carb and hit the "prime" switch. If gas comes out, you're not down to the minimum level. I'm having trouble with mine at the moment ('02 Onan Emerald 5500). It fires, but spits out great gobs of white smoke then dies after a few seconds.
Due to medical issues, it sat from mid-December past year to early May without being exercised. I think the carb float valve is letting fuel into the float bowl and flooding the engine. New plugs are already black with carbon.
We don't use the genny except when we need both a/cs, which is very rarely. At 8 years old when be bought the rig, the generator only had 48 hours on it and it's only 50 and a bit now.
Daughter and room-mate are using the MH for a base camp at a soccer tournament - we should get it back Saturday, so i can work on it some more!