My rule of thumb is, "if you don't know how to use it, there is no sense in having it with you!"
Being an amateur radio operator, I have fittings for coaxial cables, a soldering tool, some coax cable, electrical fittings, crimping pliers, wire strippers, antenna masts, antennas, wire to make antennas, etc.
As for the chassis of the motorhome, or the coach itself, problems are so rare that other than a few fuses and light bulbs, I don't have much repair stuff for them. When a problem does arise, it is easy to make a trip to a hardware store or auto parts store to get what I need.
While "Be Prepared" is an admirable sentiment, there simply is no way to have everything needed to cover every eventuality. One could run out of room to stock food!
Another good rule of thumb is; "If you have carried it for 2 years or more, and have never needed it, it is probably safe to leave it at home and free up the space!" Of course, that does not apply to EVERYTHING; common sense comes into play, also. The spare tire, jack, lug wrench, and fire extinguisher, for instance. Even though I have never used them, they stay aboard. But then, the space they occupy is unusable for anything else anyway!