I've never used a tank, so I can't speak for how to raise your humidity more than you are. Two water dishes and misting the tank are two ways you're attempting to, and if it's still not working, I'm not sure what else to try, so I'll let someone else chime in there.

If you do decide to eventually switch to tubs, I will say that humidity has NEVER been a problem in tubs for me. In fact, I've had to switch to a very small (4oz) water bowl for my BP's tub because the larger bowls had my humidity up too high!