Where is the lever to bypass fresh water tank?

Is your water tank under the dinette seat just behind the drivers seat ?

The water fills should be independent of each other. The city water should not fill the water tank.

On the exterior of the 26Q, there should be an access door with a keylock located above the generator compartment. That's your freshwater gravity fill port.

city water inlet should be on the drivers side above the rear wheels.
On some models, its located on the passenger side with close access to the kitchen area.