These are some great tips from wilderness.org for tent camping,and make perfect sense for truck camping.
KEEP TOASTY AT NIGHT
Double up on sleeping gear. To stay warm, use two foam pads instead of just one between your sleeping bag and the ground (plywood). You can also double up on sleeping bags, or put your bag in a bivvy sack.
Use a bivy sack. Put a bivy sack around your sleeping bag for added warmth. This can increase your sleeping bags capacity by up to ten degrees.
Remember, the hot water bottle is your friend. Snuggle up with a warm water bottle in your sleeping bag or place it at your toes to keep your feet warm.
Eat a snack just before bed. Your body stays warmer by burning calories, so a snack full of carbs just before bed can help increase body heat. Don’t worry about the calories. You’ll be burning them off with all the moving around you do while camping.
Do some jumping jacks. Warm up before getting in your sleeping bag by doing some quick exercise, like sets of jumping jacks, just before bed. This will warm your body and ultimately your sleeping bag. Just be careful not to exercise to the point of perspiration as you do not want to introduce humidity to your bag.
Do not breathe into your sleeping bag at night. It may feel good to bury your face, but the condensation from your breath will add humidity to your bag. If your head is cold, wear a balaclava to retain body heat. It will warm your both your head and your body.
Force yourself to use the bathroom before bed. Just like mom said, even if you think you don’t have to go, just do it! The last thing you want to do in the middle of the cold night is crawl out of your tent (truck) to empty your bladder.
Do you have any tips about sleeping warm while camping?