Have had this problem with the truck since I bought it. If it gets left outside overnight even at 0'C it will not start unless it's plugged in. I originally thought it was GP's so I changed all 8, still the same thing. I only get about a 0.3 voltage drop across the GP relay and from what I've read that is acceptable. The truck almost always overnights in my shop but even then I still plug it in because it runs very rough after start up until it has warmed up. This last week the truck got left outside all week at a Park and Jet at the airport not plugged in. When we got back the weather had turned bad and it was about -15'C out. Tried to start the truck, no go. Left it for the night and went back this afternoon, temp had come up to about -2'C, still no start even with a shot of ether and an F250 with both batts jumped and the F250 in high idle, also using a MM to make sure cranking voltage never dropped below the magic 10.5. Towed the truck over to a plug in stall and left it plugged in for a few hours. Came back and truck started right up with no problem. I am stumped on this one guys, GP system is working properly to my understanding and batts are good, what else could it be? Also my fuel bowl heater is disconnected due to the usual. Is it possibly a fuel problem? Thanks.