The 2010 CX9 AWD didn't come with a transmission cooler? It was factory installed on my 2013.
First question: Are you sure the camper is really 2400lbs dry? Sometimes manufacturers make claims that only represent the base model, but adding options brings the weight up higher. Don't be fooled by words like "ultra lite" or "feather lite". Know the actual weight of the actual trailer. If you can't check the weight, you can at least measure the tongue weight by placing a bathroom scale at the same height as your car hitch. If the trailer is truly 2400lbs the tongue should be between 240-360 lbs (10-15% of total weight).
Here's the catch though, the CX9 can only handle 350 lbs on the tongue. Over that is not safe, and moving gear to the back to sneak around this limit is not safe either if it results in less than 10% on the tongue.
Furthermore, I'm not sure the CX9 is safe to use with a WD hitch. I would check with Mazda. It may be fine if the total weight is under 3,500lbs. I've been fine without one but my 16' fiberglass trailer is only 2,500lbs loaded (all my gear, stocked fridge, but empty water).
Thanks for all of the great info! This is very helpful. I will let you know on Tuesday how it goes when I tow it back from the deal. Its a nice 45 min ride over a few big hills.
Just out of curiosity, did your vehicle have the pigtail in the compartment behind the 3rd row seat that you plug in the Mazda wiring harness for the lights? My 2010 GT didn't have it but my 2013 GT did. The reason I ask is because if your vehicle had the built in wiring harness, its connected to the transmission and engine ECU to help the car compensate when pulling the trailer. I towed a 5' X 10' Uhaul enclosed trailer full of furniture for 500 miles and had no issues other than lower gas mileage.
I don't think the pigtail is connected to anything except your lights and a 12v source for the control box. There is no way via the 4-pin output of that box to know that a trailer is connected. I believe it just senses the signal for turn signals, marker lights, and braking, and provides an isolated output for each signal to the trailer. It's no different than the off the shelf units you can buy...Just out of curiosity, did your vehicle have the pigtail in the compartment behind the 3rd row seat that you plug in the Mazda wiring harness for the lights?