This was happening in Florida a few years back. Big diesel pickup truck hauling a box trailer parks over fuel ports, guys in trailer extend hoses into fuel ports and suck the gas into tanks inside the trailer.
Orlando Police caught a group of guys who stole like 4-5 million bucks in gas and diesel, mostly resold it to trucking companies (they don't care where it comes from).
One group was so bold that they robbed the county Sheriff's motor-pool fueling depot - TWICE. These guys got caught as well eventually.