Well the radio is powered by the coach battery, not the chassis one (at least that is the way it was wired on mine).
You don't really need to put a switch in; simply tie both radio power lines to the switched power (one of them is powered all the time). This is what I did when I connected up the 215 radio. Now when the coach is in 'store' mode the radio is completely powered down.
Our chassis battery has not lost any charge since I put in the knife disconnect at the battery.
Update: Attached is the 212 user manual.
On page 5 is the wiring diagram. What you would want to do is connect the wire from pin B7 to the supply used for B4 (e.g. tie the switched 12V to both B4 & B7). (A simple solution would be to cut both wires at the point where they joined the harness with the camper strip B7, B4, and the switched 12V line and connect them all together leaving the extra 12V line disconnected.)
It is possible that the switched power is coming from the coach battery and the continuous power/keep alive power is coming from the chassis battery--I didn't check to see where the continuous power line was coming from (as I wasn't going to use it).
That table looks gorgeous!
Battery disconnect knife switch on the chassis battery:
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