I never put an interrupter switch on a battery ground circuit for just that reason. Killing the power to the battery cable that supplies the starter will usually interrupt the voltage supply to the dash and ignition switch as well. Run an independent small diameter wire (with maybe a 5 amp fuse) to whatever needs a few milliamps of constant current to keep its memory functioning.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!