I don't think I have ever heard of somebody with a PVC cage have this much trouble with humidity. A few questions:

1. Does your T8 have any extra ventilation or screens?

2. Do you live in a very dry place?

3. What are you using to measure humidity? Are you sure it is working properly?

4. What are you using to heat the cage?

With wet Eco-Earth substrate alone my AP cages humidity sit around 80% humidity. I actually have issues keeping humidity low enough at times using paper towels for substrate.