I have trouble with my humidity, but that's cause my gf keeps the apartment unstable. So i can't help you there.

I wrapped my UTH in foil too and I don't feel that's an issue. Aluminum foil, especially one layer, is extremely heat conductive.

i had a thought, where are your tubs, specifically. Are they one on top of the other? is one at a higher elevation in the room? are they side by side? my thought process is that stacked, the top tub loses a layer of bottom insulation as the heat radiates both directions. also, heat rises. in florida, it is imperative that you have fans because cool air falls. i might be comfortable sitting in my chair, but the ac constantly runs cause the thermostat is 5 feet off the ground. it never gets cool enough so i waste electricity. maybe use your temp gun to check the random things around each tub to see if there is a different temp in the room. plastic can cool easily so if one is getting a draft and the other is not, that could be your culprit.

My boys dont burrow either. i have less than an inch of aspen so i have to make sure that i keep my surface temp a little lower cause they lay so close to it. i put the hide directly over it and it hold the heat in pretty well.

hope this helps.