Q: Gas odor

Gas Vapors:

Odor emits most strongly from under the hood, but is strong in the cab, and at times from under the left, right, and rear of the van. I first noticed the smell after driving for an hour in very hot sun (in Oaxaca, Mexico, 5,200 feet elevation, arid dessert). When I drove through Central America it was not present. Then, the smell began again in the mountains of Peru. Now, the odor even emits after I have been parked for a long time (several weeks) and the van is out in the very hot sun (again, at very high altitude).

Questions: 1) Could this be an altitude related problem? Is there a way to help the van seal better? 2) Could this be related to quality of gasoline in Latin America?

Other details: When refueling after a long drive, the attendant opens the cap, and high pressure vapors comes out. Also when refueling, the pump stops prematurely (as if the tank is full), and the attendant has to begin again. Finally, the van is operating at a higher temperature than normal.

My car has 120000 miles.
My car has an automatic transmission.