I recently had this same problem with my Newell. Turned out to be the Air Actuator on the Accelerator Pedal. Yes there is one there, and it has a Plunger in it that will wear and the seal will leak.
The fix was a New Unit, $200.00 at a Truck Parts store and about 2hours of my time. Check yours out. Mine was located at the Top of the Accelerator Pedal.
Great video, your a pro! I use an older unit made by Biddle and its been very successful in finding leaks on my coach and others, I call it the cosmos ray gun. Good to know about the air throttle and the year, after your adventure in Alaska I carry a new spare.
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