The thing is, I've found that obsessing over gas prices during a long drive can actually cost you more time than you save - I once spent an extra 45 minutes hunting for a station that was 3 cents cheaper per gallon, which was pretty dumb in hindsight. What actually works better is checking prices before you leave and maybe once midway, then just filling up at decent stations along major highways rather than constantly detours.
That said, GetUpside is worth having since it gives you cashback on purchases you'd make anyway, and Waze does pull in price data if you're already using it for navigation. Costco membership pays for itself if you're a regular member - but for a one-off road trip? Only worth it if there's a location right on your route; otherwise you're adding miles and time to save maybe a quarter per gallon.