In addition to adding more flexwatt, I decided to try all of your suggestions.They seem to be working great! The bottom tub is now reading about 88, while the others are within 5/10 of a degree of each other (around 89.5). Moving the fan so it's blowing down does seem to help regulate temps better throughout, and I moved the probe where you suggested as well. Below are pics:
I know the temps are a bit off in this pic, but that's because I took it after having the door open for a long while while adding the additional flexwatt, moving the probe, taking pics, etc.
So thanks again for all your assistance! It seems to be proving very helpful!