Loren may be correct. The fumes are more apparent when the tank is filled and/or when the temperture is higher in the summer. A leak at the top of the tank sound logical. Of course my wife, like most women, has an acute sense of smell and is very quick to let me know about it. I'll snug the bolts on the gage but will be cautious about pinching the gasket. I'll also check the hose clamps. All of the hoses are rated for gasoline so I don't believe they are the culprits and the outboard does not appear to leak any gasoline when tilted.
I had the same thought about staying with the plasitc tank but found that I spend so much time in the cockpit -I hated to be bothered by it visually and it just got in the way.
My plan to add a power blower is fairly inexpensive. Westmarine has an explosion proof blower fan (by Rule -of bildge pump fame) for about $30. I plan to mount it at the lowest point below and run a 3" diameter hose along the starboard side to exit the boat at the rear though a vent. This will at the same time eliminate any battery fumes and sulfuric acid smells. Maybe $60for the added piece of mind. I think I read somewhere that smaller boats were not required to have power blowers prior to 1980. All in all, I estimate that I spent about $300 on the tank but the added value and safety have been worth it.
Good luck with whatever works for you.