Yes, I saw your mods on IRV2 but I don't have anything similar. The tubes for my vents don't extend beyond 1/2" into the duct either.
On the Dometic system, the AUTO fan speed is governed by the temp differential between what it is currently and what is set to - > 10F brings the fans on high, >6F on medium and <6F on low (from memory). Overriding these can increase cooling at the expense of noise - in my case NOISE. :)
I'm still looking for closable round 5" vents in a 7" frame which would be prettier then my home made attempts.