hahaha that's awesome! insane... but awesome!!
you don't pitch a tent??

When I don't expect a storm, I just use the bivy (bivouac) sack - a waterproof bag to cover the sleeping bag. Keeps precipitation and wind out and lighter weight.
Sleeping bag is most important for staying warm in a vehicle to trap body heat that rises (travels up). I use a mummy-style sleeping bag; contour follows the body so there's less air volume inside to lose body heat to. I use down sleeping bag because it gives more warmth than synthetic insulation of same weight.
Also note
water bottle that contained hot water inside my boots. Same concept as putting in foot of sleeping bag to add heat (keep boots warm and ensures you have unfrozen water to drink during the night).
For sleeping pad, I simply have a 3/8-inch thick
closed-cell foam sleeping pad called Ridgerest to insulate from snow. Closed-cell traps warm air better than an air cushion so it insulates better. Thickness of mattress is more for comfort than for warmth because if closed-cell foam is enough insulation from snow, it should also be enough to work inside a truck. However, for those who can feel the pea

under the closed-cell foam, an air mattress would add comfort. For minimal bulk, a self-inflating sleeping pad like
Thermarest is good (but pricey). For truck camping, any low-bulk air mattress would do. In fact, for luxury, I'm considering trimming a 3-inch thick memory-foam mattress to width of sleeping area and stowing it under the roof with a cargo net. Would add insulation from wind noise and be ready for camping anytime.