School him on the Dodge charging system, Landy.
Jeez, I had forgotten how anemic the old school alternators really were. 70 amps at 3000 RPM and then push that through a maze of regulator and switching, it's no wonder there's little to be had back into the cabin batteries.
One thing old school had going for themselves was that they weren't afraid to throw some copper at things, I'd bet the charging cable for the house batteries is 4 AWG. That's another thing the OP could check--How big is the wire from the alternator or switching device (if present) back to the cabin batteries?