A riding buddy of mine had a doggie dog put in her walk thru door. She rarely used the front the stall for hauling a horse. The first time she needed to use the front stall after that, she put her quietest horse in it. When she got to where she was headed, she had a horse with a bad leg injury from where the horse had braced against it and her hind leg went right thru it ! That particular door had a plastic lock outdoor. She had installed the doggie door for the same reason.  Another time before that, she had forgot to latch her back door and her dog got out and went looking for her. She ended up trading that trailer to a dealer who told her the first thing he was going to do in put another solid walk thru door back in it.

you may have already considered this (and it isn't the most energy effiecient but maybe safest all the way around??)

get some heavy plastic and duct tape hte plastic sheet to the wall, to cover over the door, leaving a little gap at the very bottom.  then prop the door open a tiny bit so your dog could push it open the rest of the way if need be.  some of the cold air will escape and it may be a bother to do it every time but....it would be safe.