Labs may look as though they have short hair, but they have this thick undercoat which keeps them warm. They were, after all, bred to go into water at below-freezing temps to retrieve ducks.

However, any dog kept indoors will suffer outdoors or in the tent at night. A sleeping pad (any piece of closed cell foam will do) and a doggie jacket of some sort are good ideas if you're out in below-freezing temperatures.

As one who has no luck whatsoever keeping dog boots on the dog, I have read that keeping the long hair between the pads clipped short and using vaseline on the pads helps. I haven't been out in the winter enough to test these ideas. Do keep the dog in good condition by daily walks that are long enough to keep hi--and you-- toughened up and in good shape!