The idea about a propane fired heater underneath isn't a grand idea, as others have said. You could skirt your rig and, if you have power to use, put a lamp underneath the water tank. If the water tank is your only worry, you can just skirt off that area. Even cardboard skirting and an incandescent bulb will keep it from freezing.
FWIW, at a past position I was directed to build an enclosure for a surface accessed potable water supply. The purpose was to keep the water supply from freezing and prevent damage to the system. The piping consisted of 3" pipe, two back flow preventers and a valve. I built a box around the piping with plywood and attached 2" extruded polystyrene in the inside. A 100w light bulb kept the temperature inside to 70f or better through a Minnesota winter.
