The 24V alternator is a Leece Neville 8SC 175A alternator with an inbuilt regulator, unless a previous owner has modified it like ours to have an external regulator.
How do you know that its not charging? No amps on the Xantrex monitor ?
If the red light isn't on then maybe you have an issue with the connection between the alternator output and the batteries.
I would check all the connections, particularly the heavy cables for tightness first.
Then use a multimeter to check the voltage at the alternator across the big cables when the motor is running.
If its over 28 volts then the alternator is working, if its not then the re is an issue with the alternator...and it maybe be only the regulator, which is replaceable.
Go to the Leece-Neville alternator page for this model 8SC,
there's a lot of information there .....