You can't assume that main stand-alone truck stop diesel prices are always better than forecourt prices.
At Palmerston North Caltex truck stop today 1.299 which is 1c dearer than PNth Caltex forecourts at 1.289 and 5c dearer than Challenge Hokowhitu at 1.249 where I can also get my 12c discount. A year or more ago the truck stop would have been 20c cheaper than the forecourt.
At least now I can check the price before arrival using two Android apps. The Kiwifuels app gives the Caltex truck stop price by clicking on the truck stop (but not forecourt prices) and the Gaspy app gives forecourt prices.
Using this method saved $8 today on a 160l fill and the forecourt was 4km closer.
You do of course have to be small enough to fit on the forecourt