My Blazer is old and it has a siphoning valve. I imagine your van does as well. I’d look into that.
It isn’t entirely impossible, but I can’t see it being worth ones time to steal gas. A 6 mpg hit (assuming 22 mpg or so) is around 1/3 of a full tank of gas each time assuming a 20 gallon tank. That’s over 5 gallons.
Have you ever gotten into your car and thought you had a full tank but was below 3/4?