smell gasoline in back of coach

Need help for my neighbot. He can smell gasoline in the back bedroom as they are driving down the road. 2006 Gulf Stream Sun Voyager, 8.1 Vortec. It has a full tank of gas before they left. The coach had been sitting for some months before this trip. The generator would not stay running this morning as he was checking things out before they left. Any suggestions or ideas as to why the smell? Thanks!!