Well if you are on I-90 it’s extremely close. [:)] In the trip I stopped there I don’t think we ever went more than 5 miles off the interstate, LOL! (Except the accidental detour to Trempeleau…)
I am pretty sure it is at the exit at Cactus Flat to Interior, SD on Rt 240. Huge parking lot, lots of room for you trailer! I remember it was about the only thing around, and well signed.
Edit: if you can string this back together, I think it is the place, zoom out to see how close to I-90: