I sleep in the back of my truck, probably 25 nights a year, between fishing and hunting trips, 90% of the time there are two of us. I pack light, and stuff things in the cab at night. A small shelf at the front of the bed is a great idea. Going to build one now.
I have 1/2" close cell foam (two cheap sleeping pads) under carpet on the floor of the bed, then sleep on my regular camping pad. Saves the knees when crawling around, and also adds some insulation/warmth. I've thought about adding another 1" of blue board insulation under it, for the colder months, but haven't yet.
I've tried the Buddy heaters a few times, but the condensation/venting/heat just wasn't worth the hassle of not melting my sleeping bag and making everything damp inside. I have thought about getting a small electric heater and running it off a deep cycle battery, and rigging it to charge when driving. A friend used to run one in his work truck to keep the tools/parts from condensating.
A tarp awning is very useful for cooking/lounging and keeping out of the rain. There are various kits that work, but a blue poly tarp tied to a roof rack works fine if there are a few trees around to tie the other end to.
Given the choice between truck and tent, its always the truck. Cleaner, warmer, dryer, faster setup, easily mobile, etc. I usually don't camp in the same spot more than a night or two however.