We had this problem and discovered it also happened during a rain storm if the rig was tilted in any direction and not perfectly level. Daniel took the AC out, resealed everything inside and out and we don't have the problem anymore. I am going to tell him about the slits that shouldn't be blocked though.
To also compensate for the heat (we've been in the triple digits for a while now in AZ), we put up 3/4" insulation over all the Southeast facing windows. Our windows are tinted but they still turned into ovens in the daily sun. With the curtains in place, we could literally smell the fabric cooking. Now, we remain cool in the rig, the rig is better insulated and we don't get that radiating heat problem that seems to completely cancel out the A/C.
Hope that helps! Good Luck!
Paula