We used foam board last year. We have concrete blankets this year. The inside temperature difference is much easier to manage.
We had RVSkirting.com in Colorado install ours. They come to you at the local campground. First day they do a rough cut. The second day they have it all done and show you how to install it. Life time warranty and the warranty is transferred if you decide to sell.
I’m currently using 1 1/2″ foam board around my trailer in Montana with a milk house heater underneath. So far this week I’ve dealt with -12F outside and it’s kept the underside above freezing temperatures. I also built an enclosure out of foam board to cover all the water connections and the sewer hose connection.
Depending on the weather you’re going to be experiencing it’s an absolute must.
I lived in an RV on a ski hill years ago and framed with 2×4 and sheeted with plywood. Building materials were a lot cheaper in the 90s…
Stopped at the KOA in Boise two weeks ago and went for a walk with the dog. Talked to a gentleman who stayed full time at the park with skirting around his 5th wheel. He said it cost just under 4k. The company came and did the measurements, installed the snaps to his RV, then came back a week later and put the skirt on. He said he was impressed that everything lined up even so though put the snaps on before completing the skirt.
He said he was happy with it and it did look professional. I have seen others use foam board but I think the skirt would be a better investment. With the foam you will have to remove and dispose but with the skirt you can take it with you if you decide to move.
I built my own system using sunbrella fabric for the skirting and rigid foam board insulation hidden by the fabric.
Easy build/mod even for me who is not much of a seamstress. I used snaps that I mounted with 3m5200 for weather proffing the hd rivets used to mount the snaps to camper body.
As someone who spent an entire Iowa winter in a small camper without wind protection I would absolutely advise getting one. Also an extra sheet of insulation under your mattress and some kind of breathable barrier on top of that to prevent moisture buildup