Rapid City, SD to Grand Forks, ND best route?

Small Prairie towns almost always have diesel pumps. Farmers use it. Gas Buddy would help you find locations.

Your planned route will enable you to visit Teddy Roosevelt NP and the Lewis & Clark winter encampment at Mandan, both of which are very interesting. If you happen to stop in Medora, ND have a meal at the hotel, best prime rib I've ever had (and reasonably pried too)!

You could always take I-90 east to I-29 north to Grand Forks. You will have no problem finding fuel that way and lots of small and mid sized towns.

US 85 to I 94 to I 29 is a very good road. Use that route a lot.
Or, plan a little time to stop in Medora for an overnight, eat at the Pitchfork Fondue and see the musical. Medora campground is about the easiest in and out but not much there. They do have a shuttle to the musical that is worth the extra.
Stop at Bismarck and stay in Ft Lincoln State Park. Tour Custer's house and the Mandan Indian Village.
Take US 83 north to Washburn see the Lewis and Clark winter camp, really well rebuilt log fort. Then just take US 200 east to I 29.
All worth the extra time. Or if you are in a hurry just do US 85 to I 94 to I 29. It is about 625 miles. There is a decent KOA on the east side of Bismarck and that is at about halfway mark.

Thanks for all the responses. We will be going to Rapid City from Sioux City. I decided to take US85 to I-94 and spend the night in Dickinson, then head east on I-94 and up I-29. That way we get to see some different area, as this is our first time in this part of the country. Our actual destination is near Emerado, ND just west of Grand Forks.

tagretired wrote:
Good Sam Trip planner routed me up thru Dickinson, ND, but there is a low clearance RR overpass coming in the south-side of town that has to be avoided.

There no low clearance on 85. On ND22 1.2 mi. south of I-94 is a 13'10" one but you'd have no problem with that.

https://member.coach-net.com/members/pdfs/LowClearanceBridgeInfo.pdf