Depending on how your underside is constructed it may be very easy to enclose it.
Most sign companies and even Home Depot sells Coroplast. You can buy it in 4'x8' sheets and that should fit under the RV fine. If your skirting is rolled underneath the RV you could "lay" it on top off the curl and screw or pop rivet it to it, and then you could screw it to the frame rails. I'd probably screw some lumber to the bottom of the frame rails and then use some short screws to fasten the Coroplast to the lumber. Remember to overlap it away from the wind direction.
Salem 29RKSS Pushing a GMC Sierra 2500HD!
Gotta go campin!