You will want some basic extras like fuses and screws, maybe some wire connectors and a wire stripping/crimping tool. Extra retainer pins for your hitch, light bulbs if your unit is not all LED. A multi-tester for electrical diagnosis.
A basic hand tool kit is adequate to carry, the Robertson #2 is a must and you will want a cordless drill along with a bit kit for those time you just need it. Something always comes up. You might also want a small compressor that will handle the pressures needed to inflate tires to 65 to 80 psi.
Keep a running list of things you think of that you want or need. It will all fall into place.