Spot the leak ...
If you've got a bay-window VW van, and it smells like gasoline any time you take a corner with the tank more than 1/2 full, then I bet I know what it is ...
The gas tank on these vans is hidden behind a bolt-in firewall, so it's difficult to find the leak, but with the firewall removed - no problemo. The only thing is, theoretically you've got to remove the engine to get the firewall out.
When we had our van's engine rebuilt, we asked the shop to find and fix the gas leak. Well, they didn't, and the van continued to smell like gas, which is why I had to remove the firewall later with the engine in-situ. It can be done, but if, like me, you like to do things right, you might find you need to consume a couple beer before starting. Here's how (Note, this is for a 1972 model, and is written from memory):
Remove the gas-heater and the charcoal canister from the roof of the engine bay (if your gas heater doesn't work, check the little circuit breaker switch on the side of the heater's relay box - resetting the breaker is all it took to get ours running). Remove the voltage regulator from the firewall. Remove the air-cleaner, and cover the carb throats to stop any nasties falling in.
Working underneath the van, undo all the bolts you can find that hold the lower edge of the firewall in place. The top edge is held in by metal tabs, as I recall. When you've undone all the bolts you can, and you're sure there are no more, it's time to drink your beers, because you now need to CROWBAR the firewall out.
The trick is, you need to bend it out towards you, in a horizontal line across the middle, to make it somewhat less tall than it normally is. This releases the location tabs and makes it small enough to come out above the engine. If you can start the bend by making a couple neat kinks on the two ends, you'll get a neater job than if you just reef on it. Remember, for safety's sake you'll want to put it back in.
Using your crowbar and gloved muscle-power, bend the firewall until the top pops out. I think there are a couple bolts that cannot be accessed, so you might need some brute force and ignorance to rip these through the sheet metal, but eventually you'll be able to pretzel the whole mess out. Now behold the gas tank.
If your leak is like mine was, it should be pretty obvious. On our bus, the plastic elbow that connects the overflow pipe to the tank had deteriorated, and the gas residue was obvious.
Spot the leak ...
I don't know what VW were thinking when they specified the plastic they used. It must have been pretty obvious that it wouldn't last more than about 25 years :-)
Anyway, I just went to Canadian Tire and got some piece of heater hose or something with a 90 degree bend and the right inside diameter, and substituted it. I'm not sure if it'll last more than 7 years, but hey.
Once you've fixed the leak, put the firewall back in and use brute force and a rubber mallet to hammer it back into shape/place. If you get it right you should be able to put all the bolts back in place. Replace the voltage reg, charcoal canister, gas heater and air cleaner, and you are away. We still get occasional calls to "vent the hull", but now it's from the aftermath of eating baked beans, not the smell of gasoline.