Should have an overfill tube. Not sure what model you have but in most cases you can actually see the tank and monitor the level...also you can hear it filling from inside and the noise from the vent tube (just next to the fill port) will change as it fills. Fill it a bit slowly, listen for the noises changing, but mostly put eyes on it. Your gauges are probably E, 1/3, 2/3, and full. From your gauge panel, time how long the gauge goes from 1/3 to 2/3 and this will give you an idea of how much more time to full.
2018 THOR Chateau 35SF
Two Labs, Bugsie & Blondie