RV's are limited to 21' or shorter and 10' high for a roughly 22 mile section of the Going to the Sun Road. Here are the specific details: https://www.nps.gov/glac/planyourvisit/gtsrinfo.htm
Here is the restricted section:
People who rent a Winnebago View are subject to this restriction.
People who rent Winnebago Revels are NOT subject to this restriction UNLESS you have a bike rack, which means you will exceed the 21' limit (Revels are 20' 5" long and 9' 10" high)