Trouble starting after 3 weeks

Hi all,after been away from my 08 tdi vrs for 3 weeks(i an a very long distance truck driver!) it was a right pain to start,it was turning over but not firing-after about 10 sec it finally fired up.This sometimes happends(but not always) after 1 or 2 weeks.In february the car didnt move for a week in -20.c and strated 1st time.Still on original battery.I'm thinking its the battery or the diesel is draining back to the tank-its parked on level ground.