Some Basics I usually make sure we have in the group;
Tools needed to do the jobs.
Tyre Puncture Repair.
Hoses.
Basic Electrical gear (Spare Wire, Multi-meter, Fuses.)
Filters (Air/Fuel.)
Fan Belt.
Uni Joint...or 2... Needed for some of us more then others
Coolant.
Oil.
HTB Grease.
Random Bolt Box.
JB Weld.
Usually when I travel on big trips I'm with 2-4 other vehicles that I know and trust and while we all have different Rigs a lot of the stuff can be worked out and Shared / distributed before we go. Like my Brother is an Elec Engineer, so he generally covers all/every electrical need. Where I take Grease and Grease gun which he uses. Another mate covers most tools, but it's nice to have your own small kit (and I take all Torx bits.)