Never had a break down yet (knock on wood) but have passed many a RVer on the side of the road with the hoods up. Stopped to help a few, mainly ones that I deemed didn't have computers and sensors all over the engine.
I know my engine inside and out, my finger prints are all over every single part of it. Have rebuilt the stater and alternator, rebuilt the injection pump, deleated every single sensor making the motor 100% mechanical and computer free. Short of a bottom end rebuild I would find the parts and fix it myself. I do carry a extra water pump, fan belt, fuel filters and a complete toolbox.
I guess my one scary moment was with my old Army truck returning from Ensada Mexico. A transmission cooler line fractured and started leaking. I stopped at a auto parts store and bought all the ATF they had determined to get back to the USA. Even though the boarder wait was over a hour and I had to start and stop the engine probably 300 times we made it to Del Mar, Ca before I called it quits.