edenrose

Here is the detail. I live in zone 8b. I have three musa gran nain. The are about 6ft tall (main stalk) with leaves up over 10ft. I built a green house around them (basic wood frame with 6mil plastic cover). My goal is not to grow them through the winter only to keep them alive. The winter has started off very cold. Temps daytime outside mind 50s inside greenhouse around mid 70s, at night outside upper 20s inside using propane heat ive been able to keep it in the mid 40s.
Here is my question the leaves have begun turning yellow. Does that mean im loosing the battle or are the plants lacking something? I have slowed some on watering but the inside of the greenhouse is very humid with lots of moisture dripping down. I have piles of mulch up around them as high as I can without covering the 8 kids. Could the carbon monoxide being given off from the heater be causing the yellowing? Or could it be less sun light causing it? Playing out the worst case scenario if I loose all the leaves and keep the plants from freezing will they continue to grow this spring or will I have list them?
Any and all advice is greatly appreciated.