It has always been my experience that if I had a spare anything, the one in operation would break. Case in point, had an '89 GMC S15 Jimmy with `160K miles on it, brother says "Better put a new fan belt on it, the old one might break!" So, I did. Sure enough the new one broke! 1000 miles away from home.

But... I kept the old one , so I had a spare. It was still on when I traded @ about 200K miles
I only carry a spare tire, a few fuses, bulbs, and things that I have replaced that could be made operable, or at least had usable parts on them. Can't carry everything, and I am not smart enough to figure out what is going to break next.
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'01 National RV Tropi-Cal, Ford V10, '01 Suzuki GV 4X4 Blue Ox Tow Bar,300 Watts Solar, 2500 Watt '458' Inverter, NO TVs