All of the second units I've seen have their own controls and thermostat on bottom face of the unit. A pre-wire typically only includes the 110V wiring, unless you're in one of the super high-end units that are made for full-timing.
To install a wall mounted controller/thermostat, you'd need to pull wiring for it. Not a problem in most coaches to pull an interior wall panel to run wiring, but it's not for the faint at heart. We've got one separating the bedroom from the bath, and it's been removed a couple times to run wiring from the attic to the basement... YMMV.
All that said, we're on our third RV and never have seen a need for a second unit when the main is a 15K. And we did a lot of camping in AZ/UT...