lows below freezing, bad idea to camp?

Propane puts out a lot of moisture as a byproduct of combustion. My suggestion (which will undoubtedly draw the naysayers) would be to purchase a coleman white gas lantern. They put off a lot more light than the propane lanterns and, if you intend to use them as pre-heaters inside the trailer, they won't put off anywhere near the moisture of a propane lantern.

Leaving windows cracked or a vent open is still a good idea.