Not in Montana. Actually, it varies by park. I do not believe there is, nor can there be, any flat answer to a question like that.
I take our 32 foot Class A to many parks around here, no problem.
A 36 footer may have trouble in some spaces, and a 40 footer or larger might not fit at all. However, there is often at least one space where a big rig will fit. It is often first come, first served, though, so it may have a tent or a truck camper in it when you arrive.
Good luck.
This site had good information:
http://www.forestcamping.com/dow/list/nflist.htm
