I started off with a $50 mechanic's tool set from harbor freight. That is the baseline for sockets, screwdrivers, pliers, cutters, hex wrenches, torx bits. Another small tool box (I call "electrical") for add on tools like a cordless screwdriver (which doubles as a light duty drill), soldering stuff, crimp connectors and crimper, work lights, test light, multimeter, gloves. Then a third toolbox that contains parts and spares - fuses, relays, lamps, wire, tie wraps, tape, plumbing fitting assortment (plugs, unions, interseries adapters), hose clamps, screws, nuts, bolts. It's worked out that I've helped other campers as much as myself which I think proves out the technical merit of what I've brought. I consider the goal not to be able to fix every single failure or issue but have enough resources on hand to at least create a workaround until a proper repair can be effected. Example, I've had both the toilet valve and the vanity faucet split and go geyser mode. I don't carry spares of those but just having piece parts to cap off the supply line is enough to get you by until you can get somewhere you can get parts. Have yet to have a trip detoured due to a failure.
Mark B.
Albuquerque, NM