Do you have a heat duct down there ? or heat pads on the tanks ?
Maybe wrap electrical heat tape to the dump valves ? When it gets down to 11F or the single temps, you might need a little more help than an underbelly cover that's insulated.
I have a heat duct heating my underbelly with a covered belly and no insulation . I would not want to use it below 25F.
I would foam it, add pads to the tanks and wrap the valves as well as any exposed lines. If you choose to use boards, tape the joints for a tight seal.
Remember in order to make underbelly repairs down the road, all that work will need to be removed.
Here is more underbelly insulation ideas.
https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/28616062/srt/pa/pging/1/page/1.cfm
