OP: You can do a few things to boost humidity.
1) Cover 90% of the screen top with plexiglass, a damp towel, or taped down tin foil
2) use humidity retaining substrate (coco husk or cypress mulch)
3) Get a wider water bowl. The larger the surface area, greater the evaporation. (And move over the hot spots)
4) If you have a lamp, get rid of it. It sucks humidity out like a sponge
5) Make a humid hide. Get a plastic Tupperware and cut an entrance hole into the lid. Line the Tupperware with damp moss or paper towel. This will be a humid micro-climate where the snake can go into it whenever it needs some extra humidity. This will save the effort of trying to maintain the whole tank.
You must have one really healthy tub ;P
None of mine will drink or eat.