I've heard you don't want a "rear flap" (near the very rear of the coach as opposed to behind the tires, and all the way across) too close to the road surface. I thought that was so you won't kick debris up when the coach suspension drops the bumper down. The Vacuum Idea is interesting.
We don't tow that far, but I haven't found chips. Our Class C lost one of its OEM flaps. When I made the replacement, I made two out of a tractor/trailer mudlap (FAR less expensive than an "RV" flap...) and they extend about another 2" toward the pavement. Still not long enough to kick anything up. There's about 10-feet from flap to bumper, and I don't know what goes on under there underway. No evidence of stone chips, but if it rains underway the toad gets filthy.
Three of those 18-wheeler flaps would create one big flap all the way across at the bumper. Reading the Vacuum Idea, not sure I want to do that.
