Just like any tank.......waste water holding tank, fuel tank, fresh water tank etc. they need a vent so that as you add fluid in the air can get out of tank as it is displaced by the fluid and so that air can get back in as fluid is drawn back out.
Some fresh water tanks with gravity fill have a vent line that terminates inside the gravity fill tube hatch.
Some have a vent line that is just attached high up on frame
The fresh water tank should have a vent connection on top of tank..just run a small hose up above tank...no kinks/bends/low spots so water can't get trapped and block the vent action
