I would agree with others and say it's probably laying along the highway somewhere. Seen a new RV spare in the ditch yesterday along 135 North.
Neighbor works for highway dept. He finds spares, bumpers and even a portable generator. He's found wallets, cell phones, and tablets as well. Turns it over to highway patrol. Most are never claimed.
Since the mount was gone as well I would say the bolts vibrated loose and simply fell off.
Forgot to add that I personally lost my license tag off my new 5th wheel when the cheap plastic bracket off the tail light broke off, first trip out.
Talking about a hassle to replace.