Effective ways to heat a high tunnel

Hey all.  I need some tips.  We have 3 high tunnels 70' x 30' x 12'.  We are in zone 8a.  We just had our first hard frost, got down to 32!  We tried to put an old BBQ in the middle of one high tunnel, in order to help our tomatoes get thru.  Warmed it up a couple of degrees, made a lot of smoke.  Still had frost form on the inside of the plastic.  What are some good (and cheap) ways to help these things stay warm?  

Hard to imagine that at 32F it would freeze inside the high tunnels. Are you measuring temperature inside and outside? Do you know how low it got at the plants? Frost on the walls doesn't mean your plants are in danger.

At those temps I just cover with light-medium row covers. Where I am that gets me down to about 26-28F depending on the wind. And yes, this is for trellised tomatoes. I just kind of wrap it vertically on the sides. If I had a high tunnel I would not do even that.