Check the body/wheel wells just behind the rear wheels. Most have a short mud flap there -- you may see where yours were attached if it had them at one time. You can buy a truck mud flap at a truck stop and cut it down to the shorter lengths you need. You probably should have flaps there. Look at some other coaches and I imagine you will see flaps in that position.
If you do decide to install the solid flap all the way across the rear, beware... My coach has one, BUT, all I have read about them indicates they can cause more damage than they prevent, particularly if they are mounted to hang too close to the road surface. They need to be an absolute minimum 6-8 inches or more above the road surface when your coach is at ride height. From what I have read, the ones that cause the least damage are the "hula skirt" type (NOT "solid"). If mine is ever damaged or comes off for some reason, I will not replace it. And if I ever drive to Alaska, I will remove it before I leave the US. They are a great place for mfg advertising ...
We also have a solid guard that mounts on top of the tow bar. Ours is Blue Ox, but Roadmaster and others also make them. Probably the most effective type would be like the Blue Ox "underskirt". It is a stretch cloth that hooks onto the corners of the RV and onto the corners of the towed car under the towbar, completely covering the area between the two vehicles so nothing can come up between them. Again, several mfg's make them. The primary limitation is if your coach is diesel and has a rear exit exhaust pointed downward, that can melt the fabric. I know a few owners whose setup works ok with rear exhaust, but mine would not -- just depends on it's position in relation to where the fabric is. If the fabric could be mounted to go over the exhaust pipe rather than under, that would work.
Paul (KE5LXU) ...was fulltimin', now parttimin'
'03 Winnebago UA 40e / '05 Honda Odyssey toad