Skirting

I just finished skirting mine, but I am in the Florida Keys and don't need insulation, so I just used treated 2" x 4"s for the framing and plastic lattice for the facing. It is a bear to get it level.
I also put in three hinged doors made out of the same, (good storage under the trailer)

If I was up north, I would frame it with treated 2" x 4"s, 16 " on center and then use either aluminum or plastic solid panels. Then use rolled fiberglass insulation on the inside. It would have to made so no critters could get on the inside, since cats and raccoons will pull down the insulation, unless you cover that with OSB which will be costly and a pain

I made up my framing first and then fastened it, by drilling pilot holes in the bottom member for 60 penny spikes to be driven into the ground ever few feet. Then I screwed self tapping screws through the top member into the bottom of the curved aluminum trim on the sides, the bottom of the rear bumper and across the bottom of the front trim behind the tongue. I drilled pilot holes through the member, and then tiny pilot holes in the metal.
After they were installed, I finished the face using screws through it to the members.

Jack L

We took an insulated truck tarp... folded it in half and cut it down the middle. Used it to skirt all the way around our 37ft motor home. It went just behind the tires in the front and just behind the back tires.

Where we had bay doors, we put the fasteners that matched the grommet holes already in the tarp. Closing the bay door just helped hold the tarp more secure.

Easy to unhook and fold up. Easy to keep clean, too.

You might want to use hooks and bungee cords.

Concrete insulated blankets work well, there are even some companies making a skirting system using similar.

If it was my money, I'd use extruded foam board and fashion a holding system to keep them in place. Another measure that appeared to work well was to use heat lamps or electric heaters underneath. A member of this Forum was work-camping in ND using the above system and was able to hold 65f under his trailer in ND Winter climes. He had a temperature sending unit attached to an axle so was able to monitor the temps. The only downside he had was the growth of weeds underneath and was concerned that the growth might come in contact with the heat lamps. We discussed vegetable crop harvest and rotations.:)