I use either a single air mattress (cheap one from walmart) or will use my backpacking air pad with few layers of foam, taped together so it doesn't separate as you roll around.....both work well, I think the cheap air mattress is more comfortable, but not as durable with punctures
I keep my gear in a two large tupperware containers, 150L i think from costco...so they go on the ground beside the truck and i'm in bed in a few minutes
propane heater will cause a lot of condensation, even with the insulated/carpeted roof, condensation is still there just doesn't drip down, i've turned the roof carpet to ice in my last canopy.....i only use the buddy heater to heat for 10-15 mins at a time, usually going to bed or waking up
through the night I burn candles to help take the edge off, I like the double size tea candles and will burn 4-6 of them (always light 2 fresh ones just before I fall asleep so they will last almost to morning), space them out and prop up my boiling pot over them, so it helps radiate the heat a bit and cuts down the light while sleeping....the candles keep it just above zero, coffee water doesn't freeze....I can truck camp comfortably down to around -10, after that I'll take the time to setup the tent and wood stove
I have considered using a cot, or building a plywood platform, but I prefer to have the extra head room (my canopy is flush with cab height) and my system works well enough with the large tupperware, one for hunting gear, one for camping gear, quick and easy when I want to sleep
lots of ideas in the overland truck forums/groups if you are thinking of building a deck system of some sort