Having the RV home for a sec I took off one of my forward A/C vents and found exactly the same thing everyone else has (and what halfprice mentioned a few posts above): The channels don't stop at the vents and continue forward at least as far as I can stick my hand in. On our Axis they probably are attempting to cool that big, empty, uninsulated space in the front cap.
I didn't have any expanding foam handy, nor any dynamat or anything else. So I reverted to using the Mythbusters favorite building material: Duct tape.
I used about a dozen 6" strips of duct tape to fashion a ramp to direct the air out the vent and stop it from going forward. With the vent back on it was really easy to feel the difference between the untaped vent and the taped vent.
It doesn't look pretty, but it works:
(With the vent on you can't even see it.)
I proceeded to repeat the procedure for the other 3 vents at the end of the duct work.
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