If you use some insulation on the floor and sides, and a way to circulate the air, it should protect at much lower temps than 29-30*. I used a small ceramic heater, when they first came out, and it kept the Astro warm in even the coldest weather. The only problem was condensation on the cold metal sides of the truck.
I used to use a radiator type heater in my box truck but i found that on the coldest night, the heat would rise right off of the heater and right out the roof. My hose reel near the far corner would freeze up or come close to it. And the heater would run continuiously sometimes. I switched to a ceramic type that moves the air around and it has been much warmer in the truck.