I agree, it should work just wiring the 12V side.
On my 2002 Windsor the 110 volt circuit is controlled using a 12 volt relay that then supplies power to the water heater. You may be able to do use a standard switch to provide power to the water heater so it can be controlled since I believe the 110 circuit takes priority.
Also, my water heater has a 110 V switch that you can also turn off to prevent the water heater from accidently being turned on from inside the coach. I flip this off whenever I winterize/drain the water heater.
2002 Monaco Windsor 38 PKD
2005 Jeep Grand Cherokee w/5.7 Hemi