Which Endurance do you have. If it is a 3556, the kitchen drain is under the rear half bath. Did you open it? If not, this is why your gauge reads 2/3 full.
Could be material stuck to the sensor. Each sensor grounds when water reaches the sensor. If something is stuck on the sensor it could ground the sensor causing the light to be on.
I had this in my Kodiak Cub. It took filling the tank and draining twice to dislodge what ever it was.
Paul Bristol
If that does not work try traveling with 3/4 tank and a little bleach in it.
Then drain and rinse a few times to get all the bleach out. There is fresh water sanitizer chemical on amazon which work instead of bleach and also they have a chart for the amount to use for the tank gallon capacity .
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Mike & Emily-Sacramento, CA
Our 4th trailer (2016 Aspen Trail 1900RB)
2018 F150 STX 2.7 Eco Boost