I think the best NFS campground for big rigs like yours between Buffalo and Ten Sleep would be Tie Hack. No hookups (you won't need AC), but it's fairly new (13-14 years old) and has sites that will easily accommodate your big MHs. You can reserve sites now, but the ones not reserved go to first-come-first-served within a week or two of the planned stay. Sunday-Thursday you can usually find a site if you're there by 4.
If you want hookups, Buffalo has some nice commercial RV parks -- Deer Park, KOA and Indian Campground.
We also enjoy camping at Devils Tower. Our preference is usually the Parks Service campground inside the monument -- Belle Fourche Campground. But there are no hookups, so if it's too hot we prefer the KOA just outside the entrance.
