I'm not familiar with the setup on the Jayco rigs, but I had the same problem on my Bighorn. It used the single Anderson valve to select between city water, pump, and winterize. Over time, it became very stiff and I had to use pliers to change the settings. That's when I had the exact same problem, with my tanks filling to overflow when on city water.
I did get a kit to replace the internals of the valve, and it solved the stiffness problem, but the overfilling problem happened whenever I set the water pressure regulator to 40 or higher. They do sell a replacement that uses a different design, but it wasn't available at the time I needed it.
Hope this helps.