It really sounds as if air is getting sucked into the suction side somewhere. One place to look is the screen filter - probably attached to the pump on the suction side. Look for a loose filter bowl, or a missing bowl gasket. Also as previously suggested look to see if the winterizing hose valve is slightly open.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch