I've had this problem with a rental that was always used on hookup/city water. The vent line was installed wrong with a dip that filled with water. I cut the vent line, reran to get rid of the dip and reattached. I bought a used trailer and the vent line was clogged because of mold. I replaced the vent line, cleaned everything and completely flushed and drain the fresh water tank many times.
I'm assuming the hose was going to the fill port, not the city water hookup. No need for the pump to be on.