Has it displayed full in the Past?
It is 2020 so it is still under warranty. Have you reported the issue with Thor? Yes you may figure out a way to fix, but nothing like reporting it in the event it needs to be returned to a Dealer.
But if you are determined to fix yourself, you may need parts anyway. If you can confirm by eyeballing and tank is 100% full, then you will need the specs to test the sensors to ensure they are working or need to be replaced.
2019 Sunstar 29VE Sumo Springs; Safe T Plus; EC-30 AGS Nighthawk X4 AC2200 WiFi EX7300; H/W Heater / Chassis Disconnect Switches; Southwire Surge Guard 44270 & 34951 w/Monitor 40301; Jet Flo Macerator; Alpine SPE500 Speakers; Visio M21D-H8R