The Fuel filler neck would/has never been the supply point of the On Board Genset. I find it hard to believe they would take the time to completely remove the Genset supply hose when they removed the Genset. What I could believe is, in 13 years at some time the tank was dropped to replace the fuel pump/sending unit and they did NOT install the one for the RV side of a fuel tank which has the separate down pipe that stops at the 1/4 fuel level. They did this because they no longer had a Genset. But, OEM Gensets on Class C motorhomes ALWAYS have the 2nd pick up nipple on the pick up of the Fuel sending unit and they are ALWAYS at the 1/4 fuel level. Have you traced the vent line at the tank to verify it does indeed go up front to the Engine? Or is it capped and tied up close to where the Genset was? Doug