Sounds like something similar to a project I installed years ago. I have a 12 VDC switch on my dash. It is wired to a solid state relay at the rear of the bus that controls the engine block heater (110 VAC). I used a Crydom TD1225 solid state relay. The input side accepts 3 - 32 V Dc to close the circuit on the output side (maximum of 25 amps.) Works like a champ. I mounted the relay inside a standard sized electrical outlet box.
On cold mornings I used to have to go outside, walk to the back of the bus, open the tailgate and plug in a cord. Now, from the warmth and comfort of the bus, I just flip the dash switch.
Now this post was a piece of shear genius. I do hope you don't mind me copying your engineering!