Personally I wouldn't heat a van.. it's not required and if the heater goes on fire.. your insurance won't pay out.. I know it's a very small risk.. but it it one worth taking for little or no benefit ?
To protect from frost damage, drain down the hot and cold lines, fresh and waste tanks.. leave the taps and drains open.. not forgetting the shower mixer.. drain the hot water tank.. Leave the locker doors open.. make sure there is adequate ventilation and you won't have any problems..
I also have the EHU cable connected and put the battery on charge for a coupe of days every two weeks or so..
Had a low wattage greenhouse heater in our caravan few years ago as constantly on added £100 to electric bill in one winter! Replaced with cheapie Tesco oil filled radiator & kept on frost setting so would only cut in when really cold & dehumidifier set to cut in when quite damp - much lower running costs & kept it dry & aired. Didn't feel the need to use either in the motorhome last winter as stored in a better well drained spot & it has plenty of ventilation built in which worked even under cover as it's a breatheable one
We are always on EHU as the van is in the drive, so we just set the heating on low and forget about it, stops the boiler auto drain tap kicking in as well, mind you for the worst three months of the year were in Spain.
Hi we use a 200 watt tubular heater over the winter with a timer, set to come on about 4 am and off around 10 am we have found that it reduces the damp cold feel of our 2nd home, regardless of if you need it or not we find it keeps the van ready to go as we use it all year round and occaisionally when were not using it I will put the air contitioner on at 23degrees for a couple of hours just to "warm the fabric of the van" there is no science behind it its just what we do - hope this helps