I would construct a 2x2 frame and friction fit it to the 5'ver and the ground. Maybe a brace here or there then 45 degree into the ground under the trailer. Insulation board on top of that, and if you're going for looks then a stucco on top. under the 5'ver I would run a string of lights or a barn heater on a switch. On the really chilly nights you could turn on the switch giving off some heat in the area below the trailer.
If you go to the big box store they already sell a foam board with stucco on it for the exposed outside of a house basement