Most often this mixing of inflow and outflow air happens in the box BELOW the rooftop air conditioner, accessible by removing the plastic inside grill cover by removing the filter, removing the screws under the filter cover, and removing the 2 other grille screws under plastic caps on the outlet side of the grille. Use metal duct sealing tape (not regular plastic duct tape) to seal all surfaces contacting the air flow on the intake side for your humidity problem, and on the outflow side (into the ductwork) to prevent cold air loss through leaks.
Also, in any appreciable humidity, you should see a puddle of water on the roof around the AC roof shroud after running the air conditioning an hour or more. There are several drip holes for the excess condensation in the metal base of the AC for this. These can get plugged with dirt and debris, so it might be worthwhile to remove the 4 cover screws and inspect (and maybe clean out) the interior of the AC unit. I drilled additional drip holes into the corners of the base unit on mine because of the possibility of other holes being clogged, or inability to exactly level the rig. I normally see condensation water dripping off of one of my corner gutter extensions when the AC has been running for a while, and a larger gush stream of water there when the AC thermostat shuts it down; as the air fans suction in the AC tend to hold some of the condensation in the AC bottom pan until the fans turn off.