As you lengthen the exhaust, you are adding back pressure to the system, and I'm sure that there are limits to this. The fewer the bends the better, the larger the diameter probably the better. I would think the system would tend to carbon up more, making it likely to not burn as cleanly with higher back pressure.
My personal solution is to have the system as clean burning and well adjusted as it can be, and I find under those conditions a slight odor on star up, and hardly detectable once warmed to temperature. If someone is sitting outside next to my coach near the exhaust area, I shut the system off. Under those conditions, it is quite warm outside, and I can live quite well without it. I do not dry camp with a crowd around, but can see the need under certain conditions, when the wagons are all circled, to extend the exhaust. Perhaps arranging the coaches with the exhausts facing each other, when dry camping, would be enough.
One of the big issues in my mind is having a very clean burning system. I have been next to some coaches where they are constantly blowing smoke out of the burner system when it is running...a lot of smoke. That is hard to live next to, even for a day...
Later Ed