I have (3) suggestions
1. check the grounds and check that fan wiring, perhaps the main power for the fan is not connected to right spot, sounds like somebody connected it to field wire for the alternator
2. fine a Hot Rod after market car parts seller, and get a new alternator that has high amps at low rpm, the one i bought for the Pace arrow on P30, was about $200
much better alternator than the ones from the car parts store
3. is more about use
does he like to dry camp ? does he spend several days or more in one place, then get up early and take off driving ?
i suggest a slight change of routine
let the engine idle for 30 minutes before leaving
or run generator for (1) hour before leaving
stock alternators, have a hard time, dumping max load amps into batteries for more than a few minutes
getting on the road at 60mph with low batteries really taxes the stock version of these delco alternators,
the stock versions are only good for about 60 percent continuous load
which takes care of ignition, heater a/c fans and lights
but over works them trying to charge (3) or more batteries that are low