In regards to the flying J you only need to purchase 20 gals of gas and that will give you 1/2 shower and 1/2 drink. you will need to go to the fuel desk and sign up for a flying J frequent fueler card if you don't already have one. then after you have the card you just insert it into the card reader first then follow the prompts on the gas pump. first it will say press 1 for cash or 3 for credit card. followed by insert purchase card,if using a credit card. then do you want a receipt yes or no. also a good thing do do after you fuel is to go inside and swipe your card through the reader in the drivers lounge and check available coupons to make sure your shower registered. sometimes it won't. if that is the case go to fuel desk with your receipt and tell them, they will make the correction. usually by giving you a coupon for a full shower. showers and drinks on your card are good for 7 days from the date of purchase. also get a booklet of the flying j travel plazas so that you know where they are all located. after awhile you'll know where most are in the areas that you run. and you will probably only stop at a pilot in emergency cases. lol also when you know where the flying j's are and your low on fuel, stop and only get enough somewhere else to get you to the J.
on gmc and chevy if your gauge is touching the 1/4 tank mark you should be able to squeeze in 20 gals.
stay clean,
Fr8 Shaker