Odor of 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 commence again.
The van is operating at a higher mean temperature than normal.