what @Adventurous said, Espar or Webasto. I have the Espar diesel and it works perfect. If you are on propane, a cheaper alternative is a Propex heater.Holy hell, froze my ass off last night. Learned the Lil Buddy heater burns through a can of gas in about 4 hours on low (only had one fucking can in the box).
I am covering every damn inch of this rig in some form of insulation. I am a little concerned about my heat source now. I hope to have my other battery tomorrow, I will set up the ceramic and see how it does and how bad the draw is.