Unless someone here knows the answer to that question, I would say since Winnebago installed the genset, they could best provide the answer. In any case, I'd make the seller have the gauge fixed/replaced before any money exchanges hands. If the gauge is malfunctioning due to a faulty engine VDU (vehicle data unit....sends all the signals to the dash gauges), that can be a pricey item ($418 plus the labor to R & R). The VDU's in some FL motorhome chassis of that era had problems.
Since that rig is built on a Freightliner chassis, a FL dealer should be able to get the fuel gauge working.
'07 Country Coach Allure 470 Siskiyou Summit #31578, Cummins ISL 425; 2014 Ford F150 toad; Air Force One Toad Brake.
Glen Allen, VA; Smith Mountain Lake, VA.