It depends what size tank you have. If you know how many gallons a full tank holds, then multiply that number by 0.84; the answer is how many gallons are in it now.

Just filled up the tank for the first time and the tank was nearly empty. I'm guessing the tank is about 12 gallons, because it took about 11.5 gallons to fill it up.

If it's the tank behind the seat, it's 18.5 gallons. There was an optional side tank offered in the camper's special edition that I believe was 22.5 gallons.