I think your overhead heat lamp is drying things out too much. If you have a belly temp of 91, drop it down to 89-90. Turn off the overhead light, then check your humidity.
I know, I'm going to hear from a bunch of people, but I don't worry too much about my humidity. I soak my snakes every week and I think that's enough.
Others will tell you that humidity is as important as temps, but I don't agree. My snakes have always been healthy with no issues.
So take that for what it's worth.
Jim Smith