I have gone down some 10% grades in my older 97 Ford Bounder class A, 30' long and 16,500 pounds with a 17,000 GVWR! It has the same brakes as the then current F-450 and F-550 trucks. THey are HUGE disk brake rotors and pads about 6" tall and 2" wide! They are much larger than any car that I have ever changed.
Anyway going down the 10% grade, I would slow to less than 40 MPH and shift to second gear, and that brings up RPM to about 3,000. At 3,000 RPM, the engine braking is significant, and would help slow the motorhome a lot. Letting it rise to 3,600 would provide even more braking. That is about the limit that I would put to my transmission. The 2005 and later Ford transmissions can handle the engine turning 4,200 RPM downhill, and provide much more engine braking than my engine, plus most newer cars shut off fuel to the fuel injectors to provide more braking and help slow the vehicle, and save fuel too!
My motorhome will beat my buddy's Country Coach with a 10 liter diesel engine up a long grade, such as the climb to Mammoth Mountain! That is because it is lighter weight, and can run higher RPM than a diesel can. The V10 can run 4,400 RPM intermittently, and 4,200 RPM all day long. This is because of the tiny 0.68 liters per cylinder. Think of my 460 V8 (7.5L) as nearly 1 liter per cylinder (about the diameter of a 2 liter soda bottle, but 1/2 as tall). The 8.3L inline 6 Cummings DP would have about 6" diameter and 6" stroke, with 1.4 liters per cylinder. The Cummings 8.3L max RPM is only about 2,500 - 2,700 depending on the model and year. It is that huge stroke length that prevents them from going to such a high RPM, and prevents the DP from getting to much engine braking unless they also shut off the exhaust gas flow.
So don't be bothered by not having the engine brake. Gas engines can have just as much braking power as a DP, in perportion to their weight.
Also when going down a hill, DP or gas engine, make sure to get out of overdrive, and downshift one more gear if required. Turning on the engine brake on my buddy's Country Coach would shift it from 6 to 4th gear, and close off the exhaust airflow by about 85% to 90% covered.
In my gas motorhome, I would shift out of OD, then shift to 2nd gear if required (after slowing below 45 MPH in my case (with a 4 speed auto). This normally provides enough engine braking that I never need to touch the service brakes going down a hill on a interstate freeway (where downgrades are 6% or less steep). It is only a really steep 2 lane road, or perhaps coming up on a 35 MPH curve that I would use the service brakes in my motorhome.
Good luck!
Fred.
