Operating as Pilot Travel centers or Pilot Flying J this chain of truck stops can be found throughout the United States (and Canada). I couldn't find much information online regarding postage stamps so I decided to send their head offfice and email. The response can be seen below.
Our locations did not sell stamps. Thank you!
Well there you have it. Unfortunately Pilot Flying J convience stores and truck stops do not sell postage stamps at any of their 550+ locations throughout the country. On a related note though some popular truck stops they run do have USPS Drop Boxes so you can still send mail from quite a few locations. I have also heard from some people that you can hand letters to cashiers if they are friendly enough and they can add them to their outgoing mail. For this of course you need the stamps already so if you are on a road trip I would be on the lookout for a close USPS or if you are looking for a gas station look at the list here or find a close Petro.