As far as large vehicle restrictions I think maybe you are thinking of US 20 going in to Sweet Home. This hwy does have length restrictions, it is curvy and a little steep in areas. The good thing is that it is only 72 miles from Sisters to Sweet Home, where it is 136 from Sisters to Sweet Home taking 20 to 126 to I-5 and then OR 228 to Sweet Home which would be about 150 miles.
I have towed a 36.5 fifth wheel both ways and it is better going from Sisters to Sweet Home on US 20 then going the uphill the other way. But if it were me I would just take it slow and stay on US 20.
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