Did you get it to work?
Sometimes I have to open a winterizing drain and connect a hose to push water in to fill the void in the hose. The pump just keeps trying to suck air for some reason.
I have not had the issue in years though and it was a real tough one to sort out.
Of course you didn't have shore water to do that.
They put the pumps in places you often cannot see of course but a camera might help look for any water lines that have air in them. A loose connection on the input to the pump might allow air to be sucked in and not pull the water through.
Certified Senior Electronic Technician, Telecommunications Engineer, Telecommunications repair Service Center Owner, Original owner HR 2008