Wow all of that and only 15%? That's weird. I have a 20 gal for my BP. Normally I keep about 80% of it covered with cardboard wrapped in aluminum and the heat lamp on the hot side. I use eco earth and mist it really well whenever dry patches appear (which is almost every day since I keep it thin so he can feel the belly heat coming through). I get 70-85% pretty easily with that. Lately I've been taking a section of the carboard off for more airflow since it's getting hotter and the ambient has risen to 86F without it. Even then I'm keeping 55-70%. Oh I also have his water bowl close to the hot side and I dump and refill the water every day/every other day.

Maybe try eco earth? If it gets really necessary what about a room humidifier? When I first got Allure (and it was much colder) I had a small personal humidifier going constantly in my room and that seemed to help. Now that it's warmer I'm not really having an issue with the humidity since I don't have to run my little space heater to help boost the room temp. Is there a vent near the tank that could be blowing on it?