Here goes...

The temps DO fluctuate a along a heat tape. Shouldn't be wildly, though.

Also, your heatgun probably measures the temp on the paper towel. Which seems slightly above the bottom of the tub, since nothing is "pushing it down". Lift the paper towel and then measure the bottom of the tub directly. Once your snake is on top of the paper towel and pushes it down, it might be warmer.

Actually, your husbandry really doesn't seem that bad ...???

RI's are usually viral. If left untreated to long or sometimes even when treated, bacterial infection can also set in. Some strains can be difficult to treat and if you don't use the correct meds or you don't use them long enough, you just create a "super bug".

Have you ever had a culture run on what organism is actually rampant in your BP's and which medicine eradicates it ?

If this was me, I would do the following:

Take 2 of the worst (health wise) snake to the vet, have a culture done on each one ! Start ALL snakes on a good broad spectrum antibiotic. Treat all of them at the same time. Once the culture comes back adjust the meds (type, length of treatment, etc) as needed. Again, treat them all.

During all of this, keep everything super clean. Clean and SANITIZE the tubs daily or at least every other day. Purchase a safe but highly effective Sanitzer like F10, it kills all kinds of bad stuff. Also wipe the rack down with it. Even though you are treating all snakes, still wash your hands thoroughly and sanitize them between handling each snake.

Might be a lot of effort and expense for a few weeks, but perhaps you will knock this out once and for all.

After all this, quarantine any new or sick snake.

Good luck