Are motorhome grey water tanks vented?

avondriver

Draining the sink is slow in our motorhome. If I take off the cover of the trap water fair floods out. Simularly if I build up a bit of a vacuum in the tank by use of a sink plunger wate roars into the tank.  This leads me to think that the problem is that air cant get out of the tank to let the water in.. Should there be an air vent - and could it be blocked ? Or would this be just too advanced for a motorhome design?