Air Bubble in Water Line Fix
Just dealt with similar problem on a week long trip. Took RV out of storage, filled freshwater tank, but the gauge said it was empty, but water gushed out of the fill spout, so I assumed gauge had malfunctioned. After a week long trip, pump acted like there was air in the line, but could not fix...water worked fine on city pressure.
After returning home, I tried the following steps for a fix and it worked for me. In the driveway, I opened all of the faucets, drain line to freshwater tank, drains to hotwater heater, and started pouring water into freshwater tank...I left my grey water tank drain open since it was pouring out nothing but fresh water! At first the water was flowing slow out of drains, and water kept spilling back out of freshwater fill....but I kept at it and had water shooting everywhere! Finally, after a couple of big gurgles, water starting coming out of the overflow spout next to the freshwater intake.
I stopped filling the freshwater tank, and closed the hotwater heater and freshwater drain and all faucets. When I checked my freshwater tank gauge, it showed full!!!!! Turned on pump, and it built up pressure and turned off. Testing all faucets showed no air in the system. I then used a fine wire to clean out the overflow vent mesh filter.
So, it turned out I had the mother of all air bubbles in my freshwater tank that would not let me fill the tank, and the empty reading turned out to be correct. This time, between trips, I did NOT empty all tanks and leave valves open -- instead I left the freshwater tank and hot water tank full, and put RV antifreeze in black and gray tanks just enough to keep the valve seals wet.
Hopefully you can find this information useful for your problem.