We tend to plan things considerably. For instance, before we left for our last trip of 5,300 miles, every night was reserved well in advance (months for most of them). We know exactly what we're doing every day of the trip, or pretty close. For gas however, I have not found the need to plan it out much ahead of time. When I get down near 1/2 tank, I give my wife an range of miles ahead I want to fill up. I like to fill up before I get to 1/4 tank. Each tank can be quite different depending on wind conditions. I had one tank that was near 9 mpg and some under 6 mpg. That drastically changes when you might want to get gas.
My wife does use google maps to check the station out unless it is a truck stop. If it is not a truck stop she is able to tell from the satellite images if we can pull in easily or not. I will say, with my Suburban and 28.5' trailer, there have been a lot of stations that would have been much more difficult if our trailer had been a 36' trailer instead. I think we would have still been fine, just would have been more selective on picking the gas station.