Stumbled upon this: http://www.amazon.com/gp/product/B008Z7R9H2
and the manufacturer: http://www.rvcomfortsystems.com/
(click on "Products", then "Ductless")
It is an electric heating element that attaches behind the front grill of your furnace and integrates with the furnace controls.
At 1,900 watts I think it would heat a larger B (like a Sprinter). I sometimes use a 700 watt electric heater and it warms up the space in my smaller sized B nicely. Seems like a good idea for when you are plugged in and already paying for electricity at your site.
The inch or so that it ads to the front of the furnace would be a problem in my B so measure carefully before considering it. Also it would need a dedicated 20 amp AC circuit so that will add to the cost and complexity of the install.
Still, it seem like a good idea if you camp with electric hookup when it is cold out.