I have found using my dirt skirt that the hitch mounted flaps can move around on the mount. The bar supporting the skirt, when it moves rubs the bumper causing a wear mark along the bottom lip of the bumper.
I am in the process of making a permanent mud flap. I am making it high enough so it won't drag when the fiver is attached. It is likely to be high when not towing but I also have regular mud flaps that came on the truck.
To make it I got some conveyor belt from brother and will get some steel from BIL. A few bolts and a couple of u-bolts, maybe a little paint on the steel and it's done.