I'm not a mechanic, but I can do some repairs. I try to think of the things that CAN happen and pack the tools and items that I might need to do the repairs that I am capable of. I don't like to rely too much on any roadside assistance plans. They sometimes take too long to get to me anyway. For major breakdowns it would be good to have however.
Tire troubles are high on the list, so I pack a battery powered impact wrench (
like this) for the lug nuts, and whatever's needed to lift the camper. I also have a 12v air pump just in case.
Electrical problems have been part of our travels, so I pack a box of various butt splices, electrical tape, ring connectors, wire and the like. Also of course a crimping tool for those connectors, and wire strippers. A multimeter is good to have, and I also have one of those harness troubleshooting devices
like thisThose things usually have gotten us out of tight spots out on the road in the boonies. But I also carry a little plumbing stuff to fix leaks in the camper. Clamps, extra hose, a length of pex and pex tools.
First aid kit is high on our list of things to keep in the truck.