UPDATE:
Well, we officially have AC again and the temp in the house is getting cooler by the second.

I am astonished to report that temp in the tanks never became an issue. The small tank crept up to 82 and then dropped back down to 81 with an ice pack in there, which we only had to do about five times over the nearly 48 hours of miserable heat. I had a plan in place for the big tank that was to go into action at 82 degrees, but it never did hit 82, only got as high as 81.5 a couple of times and mostly just stayed at 81.

We had three small clip-on fans blowing across the surface of the water (that we use every day) and also two larger fans pointing toward the open doors / open top. I guess that's what kept the temps in check.

Thanks so much for the advice, support, and well wishes from everyone and particularly the offer of a chiller from JT (gseclipse). Gotta love MAAST as a community of support!