New Camper... Fresh water doesnt drain unless pump is on?

I took home my 2014 tracer 2670 yesterday. I am trying to sanatize the water system. everything runs great until I go to drain the system.. I open up the two low water drains and the fresh water drain, also open up the faucets, problem is, the two low point drains drain fine, but the fresh water just drips.. if I turn on the pump it drains perfectly fine.. but I have to keep the pump running for the 10 minutes or so to drain it. the owners manual says to leave pump off while draining.. why would this do this? any tips? why does it only drip out of the drain when pump is off? thank you