For most adult males that 3 ft Vision should work fine. However my preference is to err on the side of bigger. My adult Female can be very active at times and likes to stretch out. I plan to move her from her 3 ft tank to 4x2x2 plastic. But she is 5 foot long and will utilize that space.

Low humidity with a Vision cage is very surprising.

Sounds like a substrate change is in order, like cypress and or coconut husk.
Maybe try a bigger water bowl first.