Oldcamper, you should be able to create a kind of "dipstick" out of a straw, piece of wire or something similar that you can bend into a right angle that you can insert under the battery cover with the top down. I'm pretty sure the tank is close to empty with the top up. It should be within 1" of the top of the tank with the top down.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew