In case OP or relevant readers are still having issues:

What kitedemon said about humidity coming from the size, shape, and placement of the water bowl stuck with me. This is the first place I've read that. Rather than drill my tubs further (there are already 2 screened in 4"x4" windows on top- 41 qt bins), I decided to attempt moving the water dish instead. I centered it on the cool end where it wouldn't have influence from the UTH. I also moved it from the edges of the tubs so there is a "track" for the snake to move around the outside of the tub without being "forced" to take a swim. A substrate change (aspen) and 24 hours later, my ambient humidity dropped to 61% from what had been 80-90%. I'll take it. Thanks for the advice, kite.