Probably because with that transmission in a motorhome there is not enough to be gained to create a demand. The answer is in the two brochures taken together.
Assuming you don't want to gear split through the whole shift sequence, the .78 overdrive is most useful at slower highway cruising speeds. So number one use is a less aggressive overdrive ratio bringing engine RPM up to where the transmission won't be unlocking the torque converter or downshifting to deal with slight grades. This means 3rd + GV overdrive on the 4-speed rather than the factory OD is the most used "extra" gear. The second most useful extra is 2nd + GV OD in the 35-50 mph range on mountain grades, a speed range where 2nd is lower than it needs to be, 3rd is too high to hold speed.
Because the 5-speed 3-4-5 ratios are the basically same as the 4-speed 2-3-4 that means 4+OD would be most useful, but is not available because there is no way to lock the 5R110 transmission in that gear. For mountain driving, you still have a way to split 3-4, so the GV could be of more use there.
The big advantage of GV for trucks is different. Truck loads are a lot less predictable, while your motorhome is always heavy. Best use of the GV overdrive is finding just the right cruise gear to match the always changing load, particularly the no load condition.
When SuperDuty Fords are geared real low (high numerically) for pulling power, factory OD is not really enough for when running light at highway speeds. For trucks with "pulling" gears (4.30 and numerically higher) running double overdrive when running light puts the over gear about where it is with "economy" gears (3.55 or numerically lower) gearing and factory overdrive.
Most of the newer 6-8 speed automatics use a selected overdrive gearset for gear splitting (computer selected automatically) and quite often have double or even triple overdrives for lightly loaded highway speeds.
