Drop your inside AC cover and look at the openings from the plenum into the ceiling ducts. You'll probably find that the openings aren't very big. I had this problem on one side of my AC. I got in there and cut out some of the foam to make the opening bigger, but I was working from the top and didn't get it as large as it ought to be. Next summer's job, I guess.
The 1st pic is looking down the ceiling duct from my bathroom vent outlet towards the front of the camper. The light area on the right is where the cold air enters from the AC unit. I have the top off the AC and you can see that there's light, but not very much. The 2nd pic is after I took an X-acto knife and cut out some of the foam. It may be hard to tell, but it IS better; but needs more foam removed.
1988 Coleman Sequoia - popup (1987-2009) - outlasted 3 Dodge Grand Caravans!
2012 Roo19 - hybrid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado 1500 Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10