Thanks all - I played some more with the existing sheleves, and found out how it was supposed to work. You're right - missing a shelf. Hmmm...a year out of warranty, and who knows where the 3rd one went - Jayco, dealer, or ??? I had assumed it was just dumb design.
Guess it's time to go shopping!
2012 Ford F-150 Eco, Max/Max
Jayco 26.5 RLS, Reese Sidewinder