Hi, I did this several years ago and it was one of the best mods ever.  The manual thermostat was always too high or too low, never just right.  I don't remember the exact model that I bought at Home depot,  but I bought one that is for low voltage. (under 24 volts) I also didn't want one with five or seven day programmable features.  It was how-ever made for furnace and air conditioner. I only use the furnace side of it. Now when I set it at 66 degrees, it turns on at 65 degrees and off at 67 degrees. This keeps my trailer where I want it. I was able to reuse one of the original mounting screw holes and only had to make one new one.

 Hi, when you buy your new thermostat, it will come with instructions as to where the two wires go. Obviously, if like mine, the furnace wires will be located on the furnace side of the base of the thermostat. The air condition side was left blank.