Rich,

The 1977 Motorhome Operating Manual section on the Automatic Transmission (p18) reads:

SUPER RANGE "S" - Used when super performance is needed for increased acceleration in traffic, hill climbing, or "Engine Braking" down-hill. The selectors lever may be moved from "D" to "S" and Vice verse, under most operating conditions. "SUPER" should not be used at speeds above 75 MPH.

And in the highlighted note:
CAUTION
Before descending a step or long grade, down a mountain or hillside, reduce speed and shift into a lower gear. Use the lower gear ranges to control speed.

It's interesting that the manual states LOW "L" may be used at any speed (it doesn't have the top speed limitation as above), and the transmission only shifts automatically into L below ~40 MPH.

Richard