I've just cured a similar problem. I know you can get fuel smells from the standard fuel caps as they do not seal the tanks. I fitted locking Aero fillers inside the original caps (they are a dead size very fortunately) - these seal and I get no smells/leaks at the front. You do have to change the filler necks etc.
The rear is a different problem. I had several sources of leaks and the smell seemed to permeate back up the pontoons into the cabin, this happended when the tanks were full or after a burst of acceleration. In fact parking on my drive, which is sloped, with full-ish tanks caused a drip from the sills.
1) The bolts holding the fuel senders into the tank leaked up the threads, I used Chemical metal two pack (the only compound I found which specially states being fuel proof) to bond some threaded rod (bolts with the heads cut off) into the fixing holes - this seals them well, you then obviously use nuts to make the fixing.
2) My fuel senders leaked under the flange and I made up some rubber gaskets to go under them.
3) Lastly I had fitted some tank breather valves at the rear of the tanks. I did this at the back as some one said you can get air expansion in hot weather which forces fuel up the filler hose and potentially out. They allow air in as fuel is used (which is important if you seal the front of the tank with proper filler caps) AND are supposed to prevent fuel escape from splashing in the tank (or roll over - God forbid!). However I found they do leak if the tank is full-ish - maybe faulty? I temporarily removed the nearside and hey presto no fuel smells or leaks even with a full tank at all now.
I do still need to vent the tank as when you open the filler after a run there is a long loud sucking noise of air entering. I will probably run a line from the vent to the top of the front filler and vent it there.
Hope this helps.