I’d plan for the worst case scenario, and that you have to park in a remote lay-by, far from civilisation. If you’re at an MSA or truck stop, then you have everything at your disposal, but better to prepare for if you have access to nothing. It does happen. Food, water, warmth, and a bit of entertainment are vital.
If you get the same truck every week, personally I’d take whatever you’d need and just store it in the truck. An electric coolbox or even a fridge would be a good investment, if your truck doesn’t have one. Even if you’re doing day runs, you can still have access to fresh milk, cold drinks, and a bit of decent home-made food, rather than petrol station sandwiches. If your truck has a microwave, then nice, but if not, a gas stove and some pots let you do a bit of frying and warming stuff up.
Water is vital, so I’d get a pack of 2L bottles and store them in your truck. I’d have a bit of food kicking around. At least enough for a supper and breakfast. John West Light Lunches are quite good for an emergency. You can always call into a services Waitrose or M&S to reload your supplies. If you’re like me, an in-cab kettle is a necessity, as tea and coffee just raises my morale.
Obviously a sleeping bag and a few pillows.
A small toiletry bag with wet wipes, toothbrush, toothpaste, deodorant, aftershave, etc. A towel incase you have a shower. Books, magazines, iPad with a sim card for entertainment.
Like said, if you have the same truck every week, and you’ll be doing one or two nights out per week, I’d make my truck a home away from home, and have everything I would ever need. You’ll gradually build-up your supplies, but to start-off you need a bit of food, water, warmth and some personal hygiene. The rest are just luxuries really, but why not have them! 