Do you have a fuel gauge sender - leaking perhaps? The two vents coming in at the back to me would indicate the tank could very easily trap an air pocket in the front. You could try storing bow down once the bilges are dry. Possibly also look at better hose clamps. Is the smell stronger in the cockpit of the boat or outside of the boat. Fuel vapour is heavier than air so it tends to sink. I quite often leave tote tanks in an under floor storage area on my rig that is certainly not air tight but when I pop the floor panel I will be able to smell fuel - even though there was nothing evident standing in the boat. Might give some indication as to whether the leak is internally in the boat or definitely coming from the breather.