How much does it cost you to run your MH generator?

You have some pretty good estimates so far but what I would do since your genset is 18 years old would be to disconnect the fuel line going into the main tank and insert it into a gal of gas and 8 oz of Seafoam and run the genset with your A/Cs running etc to fully load the genset. You will exercise the genset, clean the carb and upper section, and it will see what your genset will use under a full load.