If running on "city water" filled up your tank, you have a bad valve somewhere, or one in the wrong position. I had that situation on a previous coach, and if I recall correctly, the cause was in my water pump. (I think it was allowing water to flow backward through the pump when on city water, so the tank would fill.) Not quite sure how to suggest you trouble shoot it yourself, but any decent RV repair place should be able to sort it out.
2019 Coach House Platinum 261XL RT
towing 2016 Grand Cherokee Overland