Rads are fine IMO, they serve the same purpose of keeping the frost out.
Like an immersion they will take a while to get to temperature - so you have to get them in the van before it freezes outside.
The only advantage with an immersion type heater is the warm water you'll get for a few hours, but they are three KW's so not the cheapest thing to run.
I've got a 1kw fan heated with a thermostat and froststat - which is good at heating up cold rooms fairly quickly - I just liked the idea of having access to warm water - which could come in handy if I've got a particularly grotty first clean.
I bought a weather station from Maplin a couple of years ago, it measures everything Wind speed & direction, Atsmospheric pressure, Rainfall, Temperature in & out, Wind chill, Humidity - it has an alarm that I've set to go off when the outside temperature hits +1.0c and it transfers all the stored data to my pc. A bit excessive for my needs - but it does make life easier.
Dave