Yes, the metal truck box will be cold. as mentioned before I presently have a fitted, thick, rubber floor mat. In the past, I've had a plastic liner which helped a bit with the sidewalls. Carpet or throw rugs also will insulate the floor.
On my bunk I have a 3 inch foam pad under my bag. Insulation under you is more important than the insulation on top of you. If I need additional insulation a nice wool surplus Army blanket is wrapped over and around my bag. A fiberglass topper is much better all around than one made from aluminum if you will be camping in it. My present ARE has a glued in carpet liner in the cap. I think that was a $100 option except at the time, it was "free" due to a sale promotion.
My summer bag is a rectangular style bag. My winter bag is a high quality mummy style bag, with both a draft collar and a hood (hence mummy style). Keeping any drafts out and your head covered is paramount to keeping warm and comfortable. Some folks simply cannot stand the mummy style bags. Fine, use a good quality rectangular bag and wear a wool stocking cap on your head. Even a thin liner like those worn on the head under a motorcycle or snowmobile helmet, will make a positive difference in staying warm at night.
I can't stress enough the need for ventilation all year long but especially in colder weather. I learned the hard way, in an uninsulated aluminum topper no less. I was ill prepared, I was cold, I closed the topper up tight sometime during the night. Yes that helped, I was warmer, until,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
Started the stove the next morning, and all heck broke loose. Water was running down the frosted over windows, water was everywhere, dripping from the icicles hanging on the ceiling. By the time I realized what was happening the now thawed condensation had wicked into my pillow, my bedding and some of my clothing.
Now I am older and hopefully wiser. Ventilation is a good thing, and as long as you are aware of that, all will be well.
As far as an "ultimate" heating solution? Only safe way would be a generator set far enough away to be clear of exhaust fumes, remote start, remote fuel tank for adequate run times, extension cord and a 1500 watt electric heater.