Yes, tens of thousands of gas stations across the U.S. are listed in Trip Planner. But they are not automatically populated on the map at the same time as the campgrounds. If they were, there would be too many icons on the map at the same time.
Once the trip is built (in the app) you can add a fuel stop at any time by choosing and clicking the pin that shows a fuel pump in the upper corner. At that point, you can add a fuel stop anywhere!