When I bought my used 99 F53 Class A in 2011, the previous owner let the generator, a 4k gas Onan, sit for almost a year in Mesa, Arizona. The generator would start, but ran like ****, then it would die. With coaxing, it would eventually run, but, it would surge with changes in load. The now previous owner agreed to pay and have a mobile RV tech look at it. The tech said the carburetor was all gummed up. He replaced it with a new carb. +-$700 later and it was purring like new again. The generator then had +-300 hours on it. Now it's over 1,800. Just my experience.
Chum lee