buy the best sleeping bag you can afford, some arctic 3 star or yukon robe's are made so as you can remove a layer (bag within a bag),
as suggested before army wool surplus blankets go a long way. I had a couple sown together to form a lite bag and had a flannel liner made worked great in a cheaper bag until I could afford a more winter friendly model .
don't wear to many layers in a bag, once you sweat you are going to feel miserable real quick
Re: sleeping in the bed of a truck, I have often found that I was more comfertable sleeping under a tarp draped over the side of the truck what with the residual heat from the fire can be quite cozy.
Dragonspeed's idea with the hot bottles is great ,I just packed a real hot water bottle, many laughed at first but they sure was a few more appear in camp before long.