Adding to @mnofpeace as he has some spot on suggestions. Definitely bring some extra gas just in case. Last summer we ran into an issue in Nevada where there were several gas stations closed in a row on our route. With 60-80 miles between gas this became an issue quick. I would use a NATO jerrycan like this for safety...

Wavian Fuel Cans | The Finest Steel Jerry Cans In The World — Wavian USA

Off-Line Mapping (Gaia, Old-School Hard Copy Map, etc) for when you are in areas where there is spotty reception. We plotted our expected route ahead of time and having the GAIA maps was super helpful.

-Water
-Lithium Jump-Starter or similar (in case of dead battery)...I have NOCOGB40 and it works well.
-chargers for devices
-Zip Ties
-Duct Tape
-Cash
-Headlamp (so you have light hands-free)
-Tire patch kit
-Warm Sleeping bag/Blankets just in case.
-Open Mind and Positive Attitude


We did 8,000 mile road trip last summer and 7,000 mile the year before. Along with several 1-2K mile trips in between (we have to travel long distances to get to anything interesting to us living in S. Florida). These are the things we have found most useful.