I have the Espar diesel furnace and would recommend it. It does have a thermostat and will cycle on and off based on the temperature inside the van, although it has does seem to have a degree or so swing. A small electric heater would be fine if you left the top down but may not have quite enough output to keep you warm if the temp drops into the 20's. Also I would not want to depend only on the electric heat because what happens if you don't get a hook up spot, or want to spend a weekend a Crystal or other ski area that doesn't have hookups.
Based on the fuel consumption number is the Espar manual the furnace only used a little over .1 gal per hour on the high setting which it is usually in only for a short time while the van is getting up to temp., so it doesn't really use much fuel. I have run mine for 4 days straight and it only used 3-4 gal of fuel.
We live just South of Seattle in Normandy Park (between Burien and Des Moines) and if you want to come by and take a look at our SMB shoot me an email and we can arrange something.