With the Suburban SW6DE that we have, 120 VAC and 12 VDC operation are completely separate. Read that as "isolated."
12 VDC is run by an indoor switch conspicuously marked "Water Heater" and it lights off of its electronic control board. The Power Light comes on, along with a little "Did not Light" LED. When it lights, the little LED goes out and the Power Light stays on till you're finished making hot water.
120 VAC runs either 1. Straight to the Heater where that outdoor switch will turn it On or Off, or 2. Same as (1.) with ANOTHER indoor switch. That's what we have in our Jayco equipped with Suburban SW6DE.