The Wally is the latest receiver from DISH. It's big advantages is it's smaller, runs cooler, and uses an RF remote so you don't have to point directly at the receiver to make it work, which is a real advantage in an RV when the receiver is mounted in a cabinet behind a door. Only out put connections are HDMI and composite (yellow) for picture.
The 211Z is the previous model receiver somewhat larger, uses a IR remote that must be "seen" by the receiver. Output connections are HDMI or composite for picture.
The 211K is the oldest, largest, and hottest operating. It has the most output options with HDMI, composite (yellow), and component (R-G-B) with a IR remote.
So what you choose depends on what your coach requires to hook up to the TV's.