Dick,
We have found on our coach that when we fill the water tank, using the solenoid valve, that some air does get into the pump line. Not sure how. If we are tied to a pressurized city water connection we also find that it will not clear itself and the pump will run excessively, never being able to clear the air and satisfy the pressure cut off level. We use the pump in conjunction with the city water supply to augment the slow flow rates at the kitchen sink. What we need to do, once the tank is sufficiently filled, is turn off the city water pressure feed, turn on the pump and the air will then clear with a faucet open when it is not working against city water pressure. You just need to do this right after you fill...
Filling the tank does take awhile given the flow rate and the size of the tank which I think is around 100 gallons. I believe your pump is running due to being air bound. It is not clearing working against city water pressure and it is best to turn the pump off while filling.
The 50% level indication is most likely an uncalibrated sensor or a sensor malfunction. Can you visually see the level in the tank through any access panel? If this is due to siphoning you would be dropping 50 gallons on the ground which seems like you would redily see that and did not mention excessive water on the ground.
If you fill the tank with your wife in the shower you do that at your own risk. You are a braver man than I...
Later Ed