You may have a problem with this need. I looked at a LOT of thermostats for my 4-Wheel Eagle for the same reason. I camp in the Winter. I got a "Hunter" brand digital thermostat and it has worked well for a number of years. BUT, it only goes down to 45 degrees. I have seen other posts on these blogs of others who have experimented with different thermostats. I was never able to find one that goes as low are you are suggesting. Here's another issue. If you get one that is digital, like mine, with batteries inside, the batteries can generate a "low battery" on the display when they get cold. So, when camping at 13 degrees (which I have done), I had to warm up the thermostat battery to get it to work correctly. And to reply to the other fellow's statement; yes, things do freeze inside the camper even though we have an artic pack liner.
Chris