The 24 volt system has "floating" lines between the generator and starter. It doesn't use chassis as a current path for the 24 volts. So it sounds like something is connected wrong. These are difficult systems to understand and repair. Where are you measuring the 12.7 volts?
And do you have two 12 volt batteries or four 6 volt batteries. Somewhere you should be measuring 27 or 28 volts when it's charging correctly. If you have two 12 volt batteries and you are measuring across one battery, even 12.7 volts says the system isn't charging like it should. Provide more information please.
I've heard people say Deere made a big mistake with this design.