I have found over a lot of years boating and RVing that the only spare parts you will need is what you don't have. In my experience the most common repairs are electrical based and if you have a modest supply of connectors, 12 ga wire in black and/or red, basic tools and a multimeter over 90 percent of the on road issues can be resolved. Anything that would result in an on the road breakdown is best left to professionals.
I don't even carry a serpentine belt but do change it every 5 years as a maintenance item. This is a very low failure rate item and, on my Cat, only drives the AC compressor and alternator. Both of these I can do without for a reasonable period of time.
As has been said, basic hand tools, Duct Tape, some other multi-use supplies, good roadside assistance service, credit card, and a fistfull of dollars should handle most anything you will encounter.
Also, by not cluttering the bays with useless stuff you will be able to find what you do need. To that end I completely empty and clean the bays after every season and sort out accumulated clutter.