Well.. I have 3 trucks, I drive them all regularly..
In my Chevy, anymore, there isn't much beside the spare tire in the front or back.. I don't tote too much around in it anymore.. It gets good fuel mileage, so it's pretty well my errand running truck anymore.
In my Ford farm truck (extended cab, flatbed, toolboxes whole length of bed on either side) In the cab there is regularly a car seat for the right hand man, empty soda cans and bottles, gloves, sweat/flannel shirts, usually a small bag with a complete change of clothes, tools, phone charger..
In the back there is a fuel tank, air compressor (permanently mounted) in one side box there's usually quite a set of tools (except for this time of year when I do most my work in the shop) and on the opposite side you'll find spare parts, nuts, bolts, lubricants, grease, oil, rope, wire, chains.. and on and on and on (I used to regularly drive this truck to work, and ya never know what you might need)
My company vehicle has a service bed on it, and is just a standard cab. I try to keep everything out of the cab, in case a client needs to hop in to show me another application site, or in case I have to have one of the drivers or co-workers ride along (and it is company policy to keep it neat), but it's common to find the grease gun charger and a hammer on the passenger floor, and a briefcase and a binder full of sheets that I fill out for every load I receive. The bed is full of spare parts, a air compressor, hose reel, cables, chains, buckets of oil, and a gas can. One side of the toolboxes are full of spare parts, etc, and the other side is mostly tools.. It's a royal pain keeping the company truck in order because working in the field (and usually 100+ miles from our shop, and 30+ miles from my home/shop) you never know what you will need and when you will need it, so it seems like I carry enough spare parts and tools around to assemble another complete unit.
It used to be worse.. I used to haul a lot more junk around in any of the 3, plus have so much junk in the shop, but times are changing for me..
Brad