A cooler is almost certainly a good idea, for several reasons.
o You live in California and will almost certainly want to venture up to some of the gorgeous places in the high Sierras and Rockies.
o A rating of 5000 pounds is adequate for towing a TM, but not generous. Remember that the tow rating is reduced by the weight of all the stuff you put in your car, including passengers. And of course you need to be aware that the weight of the fully-packed camping-ready trailer is a lot more than the dry weight you find in the trailer specs..
o A transmission cooler is a lot cheaper than a transmission repair.
o Although I've found one casual reference to 5000 pound towing capacity on the Sorrento with AWD and V-6 engine, I'm having trouble finding any details on that. The reason I look for it is that car manufacturers routinely state the towing capacity number with an asterisk. The asterisk refers to some small print that says "when properly equipped". You have to hunt pretty hard to find out exactly what this means, but usually it means that the vehicle has the optional factory-installed towing package. And the towing package includes, among other things, a tranny cooler. Do you know if your Sorrento has a factory towing package?
You can find some vaguely useful words here
https://www.etrailer.com/question-236233.html
Those folks recommend that you visit a KIA dealer with your VIN in hand. He should be able to tell you, though some dealers are a little cagey (or lazy) about it Alternatively, I would write to Kia Customer Service, provide the VIN, and ask the same question. I have a 2005 Jeep, and the CSR promptly sent me a complete multi-page vehicle build sheet with every detail you can imagine. Kia should be able to do the same.
Bottom line, I highly recommend a transmission cooler.
Bill