I believe the TrailerLife Campground Directory lists State-by-State rules and regulations for lengths of RV's and number of vehicles you are allowed to tow behind your tow vehicle. It might also give Widths and as well. Worth a try.
I believe all interstates allow 8 1/2 wide. If you are in a state that only allows 8 wide, you can travel on and off the interstate for just a few miles.