I think accessing underground gasoline tanks with a portable pump is a dangerous game. If it's diesel, things would be safer.

I think what you'd want is something like this but 12v. I'm not sure you're going to find it.

Northern Tool has a few 12v pumps, mostly for diesel, but some for gasoline as well. They're all meant for above ground tanks or drums, but they might have enough lift for an underground tank if you could extend the inlet tubes. There was a news story months ago about some thieves using a truck with a cutout bottom to pump gasoline out of tanks.

f you're only really interested in one (or three) particular fuel stations, it's probably easiest to find some funds and install a transfer switch and fittings at the stations and have a portable generator stashed.