I built one about 10 years ago. I originally pulled the inside plywood and insulated it between the steel frames of the walls, ceiling, drop gate, door and underneath the floor.
after a couple below zero hunts, I again pulled the inside skin, furred out the steel framing with wood and added another ¾" insulation. more insulation is great.
Consider having 500 ½" screw heads holding the inside skin on screwed into the steel frame. The amount of cold conducted by just those screws is simply amazing! by having all those screws screwed into wood was huge in itself.
after furring out the inside and adding more wiring/lights, I added one of those 3k diesel heaters from Amazon for $100. They are indirect so they're indoor safe. because they are forced air, the air in the trailer is circulated and is a much more even heat where before I was using a buddy heater and the air would stratify to the point we could hardly breathe on the top bunk and water on the floor would freeze.
doing it over, I would screw sheet metal to the bottom of the floor framing, pull the inside floor, spray foam it from the inside and re-install the floor. This would make a tighter insulation and protect the foam from gravel and ice chunks.