A good credit card! :B
In addition to what others have mentioned...
If you're handy, a set of manuals for your unit will be priceless if repairs need to be made on the road.
A folding or collapsable table for food prep, cooking, and general handiness for outside.
Different lengths of water hose (white) to accomodate water spigot locations on different sites. I used someone's suggestion on here and bought a 50' hose and cut it into a 10', 15', and 25' sections. Oh yeah, don't forget a water filter to go in-line after your pressure regulator.
Doug