Get a cap on your truck. My parents tent camp but they sleep in the truck bed if it’s super bad weather out. It would regulate the temperature some to be in an enclosed space.
“Had a friend sleep in it with the heater” implies he did have permission…?
First thing is the mat underneath you. Thermal insulation there is very important because the ground is a terribly effective heat sink. A sleeping bag doesn’t help for that because it gets squished by your body, and the squished part has little if any insulating capacity. An air mattress doesn’t help much either, because its air space is big enough to allow air currents which transfer heat away from you. The three things that work are closed-cell foam (e.g. Ensolite, at least 3/8" thick, 1/2" is better), open-cell foam (at least 1" thick, 2" is better), and self-inflating pads (e.g. Thermarest). More info here (pages 44-47).
With that squared away, the next thing is the sleeping bag. They come with different temperature ratings, and using a winter bag in the summer or vice versa can be problematic. However, getting too hot can usually be addressed by partially unzipping the bag. Just how much to unzip is a matter of trial and error. In warmer weather, we often dispense with the bags and use just a sheet, or sheet and light blanket.
Third thing is what you’re wearing. I’m usually in my undies, but for colder weather long johns help. Two things that can help you stay warmer are a knit cap and something around your neck (even a bandana can make a difference). I’d go for these before, say, a sweater. Add or shed these items to fine tune your sleeping temperature.
And “sleep socks”- thick, warm loose socks, like those made of poly.
To answer the OP, the heater is not wasting any extra fuel because it uses the heat from the engine coolant which is wasted either way. So your question really is how much fuel is consumed by an idling Ford F150.
But keep in mind that if the ambient temperature is too low, an idling engine will not be generating enough heat to raise the coolant to proper temp and the use of the heater will only make matters worse. In that case you won’t get any warmer plus the engine gets damaged.
He had permission to sleep in the truck. It just didn’t occur to my buddy that he might roll down the windows and turn the heater on full blast to get that perfect temperature. A reasonable assumption, I think, that turned out to be wrong. And now we know.
and knowing is half the battle
On a recent trip through Tennessee, we encountered a snow storm and were stranded on I-40 for about 12 hours in a Mitsubishi Galant with a 2.4 liter engine (probably half the size of the F150). I’d guess that we ran the engine about 6 of those hours. The gas gauge barely moved.
If you have running lights (I don’t think the F150 does), you’ll be extremely unpopular.
If you can’t get a truck with a cap, try sleeping in your car without the heater on. You’ll still need good insulation on your body for the raw temperature but there’s no fabric I know of that can beat glass and metal for keeping out the wind and rain.
I’ve gotten out of my tent once or twice due to the cold and slept in the car, myself (with my sleeping bag and blanket of course!)
Get yourself a camper shell and a carpet kit. That’s what I have. You won’t need to run the truck, they’re super comfortable and they kit is light so you can pull it out when you want to use the bed for other stuff.
And bonus, apparently it makes hot blondes shed their tops! 