If you are going to remote areas( outside of phone range or days from civilisation where you could be stuck for weeks - if that exists in the US) , then whatever spares for the things that will stop you getting out of trouble - things like a fuel pump, water pump, set of wheel bearings, airlines, hydraulic lines, all hoses and fanbelts, air filters, oil filters and all the various oils. You need all the tools you require to be able to fit all the spares. Get and EPIRB and a Satphone was well. Getting stuck in a remote area could mean days or weeks of waiting for someone to get to you.
If just travelling along roads, then fan belts, radiator hoses or rescue tape, fuel, air and oil filters, then just a credit card with a fair size limit
Basically just the spares that will reduce the inconvenience of breaking down to a minimum - worst thing that could happen if you breakdown on the road is it costs you a tow to the nearest garage and a few days waiting for parts.