It's been -30C~-35C (-22F~-31F) here for the for the last couple of weeks and I can def say that you should change your oil and batteries. With the correct oil and new batteries I've started my truck at almost -40C not plugged in (had to kinda run it on the starter till it caught) But I had no choice and couldn't plug it in as I was out in the bush... From my experience it sounds like week batteries and incorrect oil....

Just my 2c...

JT