If you are using an undertank heater directly on a rubbermaid, I strongly suggest you put it on a dimmer. I've seen the temps on the surface of some brands get up to 130 degrees, and a rubbermaid will NOT insulate that, and nor will your choice of substrate. Bark will allow the heat to flow right through it.
Reptile bark is commonly used to retain humidity for those species that love it. Or, more realistically, it holds water which then evaporates under the hot conditions. Definitely add more ventilation, and move the water dish, as it is evaportation that has no escape that causes relative humidity. I'd suggest another substrate as well - aspen is great for hoggies, since they don't require humid conditions. Hell, look where they are native to - Alberta. We are bone dry here.