Tire Pressure gauge
Also, make sure you have necessary tools to change a tire, such as a 4 way lug wrench etc. (or have a roadside assistance plan)
Bottle of Clorox cleaner - To spray and de-contaminate hose bib before hooking up your fresh water supply hose. You never know what the previous camper hooked up to that hose bib.
As previously posted by another forum member, you'll need some way to store your water hoses and supplies and your stinky slinky. You'll need separate storage bins for those.
Tool bag with assorted hand tools