Spark ignition engines provide limited braking, so the heavier the motorhome the less it can slow it down. In my experience I could get the engine close to redline with a standard transmission, so I doubt controlling shift points would have helped much if at all. There is just so much you can do.
Even if you could tweak the drivetrain what's it going to accomplish -- descending a little faster once in a very long time? I'd bet it wouldn't save me 5 minutes a year. I'd rather slow down and use one lower gear if it even came to that.
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