I thought an obvious answer would be to continue using which ever method you haf been and use a digi timer plug. I picked one up from asda, just over 6 quid and ive programmed my radiator to come on periodically throughout the night.
My van however is fibreglassed out and ive padded the side panals. My water tank has a couple of silver bubble backed insulation wrapped around it and i sprayed glued a sheet to top of inside van. So my van holds the temo for a little longer. I habe it timed for midnight, about 3 and then morning for when i start.