otr - check on line for Ford’s upfitter or body builder manuals - I believe the electrical manual is separate from the chassis manual. There should be alternator output information there and/or a phone number to call.
I have a Renogy 40amp DC-DC charger on the project parts shelf. The literature for it states the input draw of the charger can be up to 1.5 times the output (60 amps for a 40 amps out model) and to size wiring accordingly.
Ray on the Love You RV u tube channel installed one and showed current measurements.
This device is connected to the starting battery not directly to the alternator.
