I do not live at either of those elevation extremes but had a very similar problem. It turned out that the back side of my lower door screen had accumulated "crud" of some sort. Possibly creosote from low and slows. The screen is pretty fine and there was no indication of this on the outer surface of the screen. I realized what the culprit was when I opened the screen a bit for a cook and the temperature rose dramatically. I took a torch to the backside from a distance so as to not cause damage and then brushed the burnt residue off with a small wire brush. That solved my problem. This was after cooking with my Egg for three years.
I used both the Maverick and Thermapen for my Thanksgiving turkey. Inserted the Mav probe in the breast and when it read the correct doneness, I verified that the thighs were cooked to correct temperature. Also with the high failure rate and misreading of the Mav it is good to have an accurate back up to be sure that what the Mav is telling you is real world. If I had to give one up, the Mav would go and the Thermapen would be kept.
Another option is to have them bead or walnut media blasted. Bare metal in a couple of minutes. Many machine shops have media blasters. I probably wouldn't have them use sand or carborundum unless they kept the nozzle back far enough to not pit the finish of the iron. Leave them a can of Pam or a veg. oil soaked towel to wipe down with afterwards to prevent further rusting before going in the Egg or oven. I have used our blaster at work on a heavily rusted pan without a problem, but kept back from the surface so as to not cause damage. This was a Lodge with the rougher finish, so I then followed up with my orbital sander until the surface was smooth. Works as good as the older premium priced pans now.
I think it was skiddymarker ( if not, my apologies to the genius that thought this up) that mentioned several years back to place the cast iron grid under the plate setter when prepping for a reverse sear. The CI is then preheated by the coals and the steak can be cooked on the regular grid indirect until desired doneness -15° or so. Set the steak aside and let it rest while removing the SS grid and plate setter with the bottom door and daisy wheel taken off. In just a couple of minutes the CI grid will be ready for that final sear. This method works as well as any I have tried and saves quite a bit of time between steps.