Unless it is magic, the ceiling vents do flow LESS when the AC center one is open. You would have to check with a velometer to make sure. The AC has only one outlet fan, so adjusting one outlet HAS to effect the others.
Also, the pressure drop in the ceiling ducting and roof cooling losses would make the ceiling vents much less effective at cooling the whole rig. You are almost better to go with a couple of fans to distribute the cold air to the bedroom, IMO and IM experience.
2007 Chevy 2500HD Classic LBZ Duramax/Allison (Stock)
2005 Bigfoot 1500C8.2 AWESOME