Thanks for all the comments guys.
I'd like to clear up a a couple of things:
1) I always do a walk around before I leave the the campground. When I did it on that morning, I felt that something wasn't right but I just couldn't figure out what it was. I check and rechecked all my usual stuff (jacks up, chocks removed, power cord stowed, etc) and all was well. I just didn't notice that the entire spare tire assembly was gone. How many of you really check to see if your spare tire is still there? I guess I just took for granted that it was still attached.
2) If it had fallen off, I'm pretty sure there would be some scrapes and scratches on the bumper from it bouncing around before it fell off. There were none of those. The bumper didn't have a scratch on it.
3) We didn't hear a sound because the A/C was running and our bedroom is in the front of the trailer - far removed from the rear bumper.
I hope this doesn't happen to anyone else. It sure put a damper on the whole trip.
Happy camping everyone - stay safe and be aware!