Also, how are these spikes created? and, if the key was in the off position, then now worries about currents running through the trucks electrics, right?
The generator windings create a electromotive force (EMF) as they rotate, through the principle of electromagnetic induction. This is the fundamental of all rotational voltage generation as discovered by Faraday/Lorentz/Maxwell in the early 1800's. When there is no place for this force to dissipate, it can grow larger then the designer of the generator intended. If it gets too large, damage to sensitive electronics can occur.
In a modern alternator equipped vehicle, this damage could
occur to the regulator semiconductors. In older, generator equipped vehicles, the possible damage could be to any connected electronics, indicators, coils, mechanical regulators, ect. The potential peak voltages from a automotive generator probably wouldn't be too great since it's so small and limited in rotational speed - most probably less then 100V.
In a Mog, any damage would probably be limited to the alternator or generator itself unless you had some kind of ultra-modern, cheap (no protection circuits) gauge, GPS or other consumer device attached to the DC line. If the key was -OFF-, yes, no worries as nothing is connected. If there WAS something connected, then we probably wouldn't need to talk about damage as there would likely be enough of a load to prevent the root cause in the first place