Underbelly insulation

Our TT is not designed for heavy winter camping. The underbelly is fully exposed with all 3 tanks showing and drain pipes etc. There is also no heat ducted down nor any tank heaters. To live in it through a good winter (temps down to teens, windy, a few snows), what is the quickest and most efficient way to insulate? Adding insulation and encasing it in plastic between the frame rails? Just doing a skirt around the outside? What are your thoughts?