Just the rubber bed mat. Should be available at most autoparts, farm (Tractor supply), or truck accessoriy place.
You mentioned tie-down and hope that is both the part attaching to the truck and the Fastguns for securing the camper to them.
Would pick up a gallon of water for drinking until you get a chance to sanitize the system. They fill, but don't usually sanitize.
Don't worry about temps, you should be good. I often camp in temps down to 0* and never have an issue. If you have an exposed sewer connection, the contents may become a poop slushi, but won't freeze hard enough to do any damage.