All gas stations get an approval for some amount before they turn the pump on. Around here, some are as low as $50 and others as high as $80. When I fill my pickup I have to swipe the card twice to fill the tank because they don't authorize enough. I'd think that a truck stop probably get's approval for a lot more since truck and RV tanks are larger. How long that initial charge stays on your card before it is reduced is up to your card company.
Many restaurants do the same thing so you can add a tip. They have no idea how much you might leave so many get approval for 25% over the amount of the bill. Normally, once you sign and the server processes the charge, it's adjusted to the actual total amount.