This the current Montana law according to my DOT book and the Internet. I have Class 8 trucks.
Montana
Size Limitations: Height, 14'; Width, 8'6" (special wide body regulations); Trailer length, not specified (total maximum combined length of 75'); Motorhome length, 55'; Combined length, 75'. Triple towing is allowed but not to exceed maximum combined length of 70'.
All vehicles over 21' (including bumpers) and 8' wide (including mirrors) are not permitted through Logan Pass at summit of Going-to-the-Sun Road in Glacier National Park. Shuttle service is available between St. Mary and West Glacier. Restriction from Avalanche Campground on south side to Sun Point area on the north side
Required Equipment: Safety chain, breakaway switch, flares or reflective signs, brakes required on trailers with unladen weight of 3,000 pounds.
Riding is permitted in fifth wheel and pickup campers only.
Overnight parking is "as posted" in state rest areas. Current April 20 2017.