Couple things right off the bat:
1) What are you using to measure your temps with?
2) What kind of heat source are you using, and what are you using to regulate the heat source?
3) What is the ambient humidity of the room you have your tubs in?
A quick way to raise over all humidity in the tubs is to tape over some of the holes then keep checking and adjusting as required. You can also move the water bowl closer to the warm side (warm water won't cause them any trouble. If it's too warm for them to drink, then it's too warm in the tub). You can also try using a larger water bowl. No hides, huh? Are you aware that the plastic hides like the ones available from Reptile Basics will locally raise the humidity while they are occupied by the snake? Just sayin'.
As for the why you have a 73 degree cool side in a room with an 83 degree ambient temp, I'd have to say you have me by the short hairs on that one. Where are the tubs located in room? Where is the vent source for that room?
Pictures always help us solve problems.