This exists, just not as a web app- your local AAA will gladly map out a road trip for you complete with the locations of gas stations and attractions along the way.
Triple-A does a lousy job of planning out road trips.
My girlfriend insisted for years on how great Triple-A was and wouldn't take any of my suggested routes (I live for road trips, I've driven all over North America). Triple-A routed her from South Dakota to North Carolina via every toll-road and congested interstate there was (Chicago, Toledo, Cleveland, Pittsburgh). I said drive down to St. Louis and enjoy a toll-free drive through the Kentucky and West Virginian countryside. She went with Triple-A's route on the way out and spent who knows how much money on tolls just to sit in Chicago's traffic. Took my route back and got a beautiful ride through Appalachians in the fall.