"Tinstar".....The heater cube and snap disk in your coach was designed to be used while boon docking, that's why it's 12 volt. Since you're talking about using 120 volts as a heater supply, you could just put a simple heater in the space/bay or not worry about it at all.
If you're looking to heat the bay while in storage, the 120 volt heater will work. If you're looking to keep it warm while camping with shore power, I think you'll find the use of your heaters inside the coach will keep the wet bay from freezing, unless you plan extended camping in weather below 20 degrees.
When I had my Diplomat, I carried a trouble light with a 100 watt bulb with me. On the rare occasion where the temps might be in the teens, I placed the trouble light in the bay. You don't say if you're looking for something for long term use, but a trouble light plugged into a wall timer would probably solve your issue.