Gasoline Evaporation of 3/4 gallon per week ???????

Good point on the "no room for expansion" however, I have looked over the fuel system on the rig and the vent tube only goes from the tank to the top of the gas fill area. It is completely closed loop. The only way I see the tank can vent is from the cap or maybe on top of the tank where the Generator picks up the fuel from. I can't see the top of the tank though.