Unless I misinterpret your post, you are descending a hill in an RV towing a (‘dinghy’ or toad’) with the transmission in neutral? I hope you mean ONLY the dinghy is in neutral.
If not, you must be suicidal.
When I was a boy, in the 70’s, nearby my house two guys in a truck loaded with wood were coming down a hill with the truck in neutral and possibly engine off and they could not control it, they did the best they could to not hit any house but at the end of the hill the truck rolled over and both died when the wood got inside the cabin. It showed how danger it was to me. Hope you think twice before doing that again.
I hate to admit it but back when I was a stupid-ass, know-it-all, mechanical-engineering college student, I used to (sort-of) do this all the time in my old, bucket of bolts, Plymouth Arrow pickup with auto tranny except that I (almost always) left the engine running and simply coasted in neutral. Of course, I only would do this in one place on my many trips home from college - Troy, NY to Roxbury, CT (all by meandering back roads that zig-zagged though upstate NY, then western Mass and finally northeastern CT).
The reason I did it was there was this big ass long hill I had to go down and the Arrow had a crappy 3 speed w/o overdrive so I had to basically floor it down the hill just to keep the truck from slowing down (yes, I know, most vehicles go too fast downhill, and you need to brake or downshift, but not that one). However, when coasting she would hold a nice fast terminal velocity without any accelerating or braking whatsoever (75 ish - yea, a bit too fast, but I was a dumb ass testosterone ridden teenager).
The only reason I would now consider this to have been “not so unsafe” was that the car was pretty damn basic so it had neither power steering nor power assisted brakes so even if the engine shut off, control would not have been compromised. Of course, being an science and engineering geek, my penchant was to experiment so, to be sure of my hypothesis, I even tried it sometimes with the engine off and it never made any difference as far as steering and braking were concerned (but yes, the ability to accelerate out of trouble was gone but that was of very little import when I was already traveling at terminal velocity). Suffice it to say, now that I have cars with power brakes and steering (and I’m older and supposedly wiser), I don’t do anything so risky, because even though power steering makes little difference when the car is in motion, power assisted brakes are kinda useless without any vacuum assist.
At idle, the engine uses very little fuel.
Not true, http://sustainablechoices.stanford.edu/actions/on_the_road/turnoffengine.html
I don’t get where people always get this idea from…