Am American, can confirm that's the symbol of our volunteer fuel truck fleet. They're volunteers who drive around in marked vehicles with huge gas tanks capable of filling multiple cars before needing to refuel. The fleet was established in response to people finding themselves stranded in the middle of nowhere without gas. The US is a big place and they'd misjudge how much fuel they have and end up miles away from any gas stations.
Some people will even get super into it and cover their cars with flags and shit. It's super patriotic and fun to watch. Additionally, the US Republican party is the main supporter of this program, so most fuel vehicles are gonna be owned by hardcore Republicans (the Dems want to end it in favor of a government-funded fleet of emergency battery trucks). As such, probably like, 95% of trump trucks are part of the fuel truck fleet, so feel free to siphon gas from those as well (if you have trouble finding the sticker, don't worry, the sticker is probably just hidden among all the other stickers and flags). You probably won't get in trouble, but if you do (like you get unlucky and siphon from one of the rare, greedy "gas guzzlers", as we like to call 'em), just explain that it was a misunderstanding. It happens all the time, you'll be fine.
That said, be careful while doing it. Some people can be very judgy about who gets gas and who doesn't; if they percieve that you don't actually need the gas then they might get aggressive. It's technically meant for people in need, but like, how many people are gonna find themselves without a place to get gas in the city? Besides, there's no legal requirement for you to prove you have an empty tank. It's illegal for them to stop you, but they might try to do it anyway. As such, the best policy is to wait until they leave and do it while they aren't looking.