I went as cheap as possible a few years ago on mine using the thinnest chipboard I could find.
I first cut a 2x4 into ~6" pieces and then screwed them in to the underside of my trailer along the edge. They were inset slightly so that the chipboard would be flush with the outside edge of my trailer.
I was at a gravel space so I dug down a couple inches along the edge of the trailer. Then I attached 1" styrofoam insulation to the chipboard and set it in the 'ditch' to hold the bottom in place, and the top was screwed into the 2x4s mounted to the underside of the trailer.
I cut small access doors with a hinge to get to my drain valves.
Holding tanks have UltraHeat stick on tank heater pads.
Used this for almost 2yrs living in my trailer. Survived good winds, and -20° temperatures without freezing issues.
