No Insulation in Enclosed Bottom

WyoTraveler wrote:
That covering is most likely Coroplast. Although it doesn't have actual R value it closes off the area. Wind blowing under an RV if moving or parked can cause the tanks to freeze. The Coroplast covering seals off the area. Adding heat prevents the tanks from freezing. My previous Jayco TT had Coroplast underside vovering and I never had a freezing problem down to about 5 above zero. I doubt it is very practical to even live in an RV below those temps.

I think you pretty well summed it up. Coroplast, even without insulation above it, is better than no coroplast. A few years ago some manufactures were touting better "aerodynamics" (smooth bottom, less air turbulence, less drag) while traveling down the highway. I doubt that it made any difference in gas mileage.