Fellow owner of 90's Sero Scotty D350 5.9L Cummins Diesel RV

If the 2 batteries under the hood are the only two, then you’re correct most likely one has been isolated for house use and one start battery.
However that is largely immaterial in figuring out the vehicle charging issue.
Sounds like diagnosis and repair is a bit out of your wheelhouse, so I’d try to find a private shop or maybe a private diesel pickup shop. Not that this is a diesel issue, but anyone who works on diesels has likely been around a 1st Gen Dodge Cummins more than a few times.