First off you didn't mention if the top of the tank is covered and how. Even a CHE will heat a 55gal provided all the heat isn't escaping out the top. You can also insulate the side and back walls (Reflectix or actuall insulation will work).

An UTH will heat the substrate up (and it doesn't need to be that deep - just enough to absorb urates and stuff) - use other methods to increase the humidity (larger water bowl - and again is the tank is covered properly the humidity will build and stay)

I used to have a 55 gal for my JCP and I only used a CHE and a UTH to heat it and it was spot on...but the top was almost completed covered and the sides were insulated.

Hope this helps