When I create routes for trip planning I make them go for about the maximum distance I would like between known gas stations (~150 miles). That way I can just open the route and see the distance.
Finding gas stations can be quite an art in remote locations. Most small towns in the west do not have gas stations.
I use Yahoo Yellow Pages sometimes to find stations.
In really critical situations I post a thread in the appropriate regional forum, i.e., West, Rockies, etc. requesting information.