Does your gas gauge work? If so you should be able to break it down my quarters or 1/8's of a tank. If not it might be a 100 gallon tank. My old 87 Allegro was supposed to have an 80 gallon tank and the previous owner put in a 120 gallon tank.
2017 Grand Design Reflection 303 RLS
1997 GMC Sierra 3500 Dually