Many moons ago I had a 1980 F600 SU34 with onan genset, had the same problem. I found out the importance of the cold/warm run switch near the carb, it seems that if you didnt have it set to cold when temps were below 40 it would flood the carb and fowl the plug, draining the float and changing the plug would put her right back in action. Cheap fix, but it worked for me.
This is CABL.com posting #314738. Tiny Link: cabl.co/mbt2A