Keith
Campground water pressure can cause unwanted filling of your tank if the water pressure regulator allows pressure above 40 psi. you can adjust the pressure with your regulator if it still works.
your water tank fill solenoid should be rated at 40 psi and you can verify this by reading the label. pressure above 40 psi will allow bypass pressure to fill your tank, although if this happens, for enough time, you should be dumping excess water out your overflow tube. the reason for allowing pressure above 40 psi to bypass the solenoid is to protect your plumbing. The other possibility as Gerald states is the water fill solenoid is leaking water into your tank and this could be related to a torn diaphragm, weak pressure spring or debris in the solenoid itself, you can check this by removing the four nuts on the solenoid and inspecting the device. In any event a rebuild kit is available to repair the solenoid.
Lastly another solenoid test, if the diaphragm or spring is bad, turn on your water pump and if it runs continuously then that could indicate a bad solenoid.
A cheap or temporary solution is to instal a cut off valve between your solenoid and water tank.