You will have the same issue with vans and cars, most cars don't have anything solid between the truck and passenger compartment, just the seat so the fumes can still penetrate.
When we made the mistake of buying a minivan instead of a decent vehicle and I had to bring gas home for a lawn mower or something I would open the back vent windows and leave the front windows down to let air flow through. I would suggest that if you do need to carry gas in the back, leave the back window open at least to let the fumes out. I suppose you could always tape some thick plastic behind the back seat, sealing it as well as you can.