Venting....

[sick472]

Barring any real small pin hole leak in the tank, I would double check the sending unit seal.

My 85 smelled like gas (tanks under the bed) for a few years due to a pin hole leak that was not bad enough to drip, but enough to wick out and vaporize. The cab wasn't smelly, just the walk around areas. It finally rusted through enough to drip to the ground...AH HA! found the stink.

I assume that the vent line you have run up and back down the cab corner exits the cab somewhere? If so, you could run an extended piece of hose from that out far away from under the cab for a test. Let it set like that overnight/for a day and see if the cab smells better. That could prove the need for a canister or not. Vapors get vented away from the truck and have no way to sneak back in.