Dan, I believe our '06 Monterey has drain lines, and would think your Marquis would also. At least as I recall, water drips underneath the coach when the AC's run, and I think I once determined that the fridge's drain forks into a tube on its way down from the roof through the rear fridge access. Your coach may have them, but they could be plugged. I would expect roof debris could block the port end up at the AC. You may want to go up, remove the housing, and check around the roof tray to see if that's the case. Watch out for mud wasps that like to build nests under the housing though; I take a can of mint oil-based wasp killer with me when up there - it's relatively safe.
Others more familiar with your specific coach may know for certain if you have drain lines or not, but in the meantime you could go check things out anyway. Just watch your footing of course. I would expect to use a #2 screwdriver up there on screws around the perimeter of the housing, but can't remember for sure.
Joel
