I bought 2 of the last 3 30″X60″ Electro Warmth 12V heated mattress pads from that they had, as I have twin beds. Electo Warmth now only makes the 36″X60″ in 12V. I had previously wired in extra 12V outlets when I upgraded my converter to a PD-9145 model.
I tried it out last weekend in the backyard. I can definitely say that having the heat below is alot better than above (ie. electric blanket). Slept very warm and cozy, outside temps at 34F and using just a ceramic heater held the temp at 64F inside.
Rather than heating the whole van overnight, it’s cheaper and easier to just heat the bed. We bought a 12v heated mattress pad to sleep on. It works really well and doesn’t use too much power.
Here is a fact you might not know: 12v Heated RV Mattress and Pads are much more energy efficient than a 12v electric blanket because of the way they are used. Your covers hold in the heat that rises from below.