Get a truck heater fitted. Eberspacher is the best.
About £800 plus vat and get them to fit an extra battery that runs seperatley off the system with a split charger, that way you wont kill your main battery overnight. I have a seperate tank in the back it takes fuel from, but proper fitted ones run from the main fuel tank, they dont use much, a liter or so overnight max.
Set the temp gauge so it comes on at about zero, dont forget you are only stopping your stuff from freezing up, if you run it on max all night you will flatten the battery and use more juice. I put mine on as I go to bed, so it only runs for about 8 hours.
Got mine fitted and it works a dream now.
Murky