Don't confuse peak draw and total draw
That mattress pad is not going to run at full draw continuously, unless you are sleeping on bare steel van floor
When on your bed aka mattress, it is going to draw what is need to keep the bed at the temp you set So it cycles on off,
our bed set over the engine, lots of cold air under there
we ran a mattress pad or electric blanket on the bed in the Safari, we had four batteries and a residential fridge, and no problems with overnight power, of course I'm not in the north east
Changed MHs , don't need the mattress heater so much now