Yes, check the vent lines for both tanks. I have had that problem in the past. Some water tank vent configurations across the Ericson fleet are prone to get water in them, especially if you let the water run into the tanks until the vents let the water out when the tanks is full. As Frank says, if you trace the vent hoses to the outlet, you can blow them out with a little compressed air.
If you are letting your tanks run dry before filling them, then some problems may occur when you try to get the lines to the pump and faucets to fill. Pumps sometimes don't get the air out very quickly and lines vary in routing and their ability feed the pump. If you have a small pressure tank in the system that fills and shuts off your pump, it can sometimes trap air for awhile before the motion of the boat burps it out. Mine usually will pump some water, but the pump gets loud when it pumps air, and takes longer to shut off.
Years ago I fixed the vent from my settee tank (22 gal) by removing 20 feet of horizontal vent line and plumbing it into one of the nearby filler hoses. Several others have done similar fixes. A quick search of the site will come up with several water tank vent alternatives.