Unless you're planning to operate all the fire truck electrical equipment continuously, all you need is one good-sized 12 volt battery. That's how I drove Ed's truck from Tennessee to the KC reunion a few years ago. If it's got the Leece-Neville alternator on it, the polarity can be swapped to negative ground by just swapping a few wires around- - - -no need to change diodes. The big DC generator that was probably original equipment and the huge DC voltage regulator is a whole different can of worms! I've got the Delco-Remy service bulletins for those items stashed away somewhere.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!