Best fuel economy speed

I think most vehicles are designed to have the "Sweet Spot" mpg wise between 50 and 70 MPH, Motor homes tend to be in the lower ragne. Mine does best at 55-60 both when towing and when bobtail. 38' Class A. bobtail I've hit 11MPG just driving under normal conditions, Usually get 8-9 Towing.

Under optimal conditions (100mph dead astern tail wind) 13.8 towing. But alas that does not often happen and I really do not like those conditions.