Your idea of stopping the problem at it's source mirrors my thoughts.
I installed a set of the "Rock Solid" mud flaps. Those used on a small motor home are actually a little long for the width of our Tundra. When installed, the gap in the middle was eliminated. The ground clearance required that about 2" be trimmed off. Actually, that was not quite enough and I cut about another inch off the second set. Even that
was not enough, so an additional inch should be trimmed. My initial idea was to allow road contact to grind off the excess but the plastic material is so tough that I guess I'm going to have to get out the tin snips again.
This is actually our second set. I had left the first set too long and the repeated flexing caused them to crack next to the brackets after about 2 years.
To our knowledge, no gravel or rocks have made it past the flaps. There has been an aluminum diamond-plate box (for the generator) in front of the Casita
body for 3 years, and there is no sign of any contact by gravel.
& Ann K.