Here's the big concern, you have no idea that CO poisoning is happening, in fact it makes you feel giddy and pretty good. While it seems unnecessary, I'm with the event people.
CO is slightly lighter than air, so by having the exhaust venting over the roof it should continue safely on the breeze like nothing happened. And the design you've come up with is similar to the Genturi systems, with the vent pipe being larger than the exhaust pipe the exhaust rushing in will draw cold air in, helping cool the vent and diluting the CO.
I know you didn't want to, but good job and I think at the end of the day you'll be happy with it.
Now-2014 Sierra 346RETS 5er BUB
Then-2002 Keystone Springdale 286RLDS TT
Nights camped in 2014-28, 2015-127, 2016-10