I try hard to plan my gas stops out ahead of time. Obviously it won't work out exactly every time. So I also carry and use "Next Exit." Big reference book ($20 something on amazon) and it stakes out each exit on US Interstates. Marked in red are stations that can handle long rv rigs. It has never let me down. Obviously if you take other routes it doesn't help much. I also do some internet research. Figure out what towns or cities you'll look to refuel and look up the stations that can handle the rig. Then verify on google earth.
CAUTION: I see many have recommended Costco. I would too...EXCEPT my two costco stations near me are iffy at best. Both have sharp turns at exit. One I wouldn't even try, the other you have to be all the way right pump to get out with a Toad. BUT i have seen many other while travelling that are super easy. and you can't beat the prices most times.