Start with the basics, download the TMs so you have a schematic, grab a volt meter and just start troubleshooting, if you do not have experience troubleshooting automotive electrical systems this is really not the one you want to learn on, you should get some help.
First thing is inspect all the wiring from the batteries to the dash panel, check/clean the grounds, behind the inst. panel, back of the engine and the cabling between the batteries and starter are a few places to look, you could have radio cables that are cut (just one example) and you need to secure/disconnect those first!
Disconnect the batteries any time you are working on the electrical system!
Be very careful 24V is a different animal than 12V a short on a 24V system can bring devastating results, wear your PPE while working in the battery compartment at a MIN.
A good inspection can often uncover many issues.