Batteries are both brand new
That would indicate where your problem is.
Those 30A Fuses are the 'Reverse Polarity' fuses and blow when battery pos/neg cables have been reversed. They protect the converter/charger system
Your NEW batteries maybe connected together correctly but the pos/neg cables going from battery bank are reversed.
Follow the NEG cable to frame ground to verify it is connected to ground.
In your picture of the 'panel' you see that the RED cable goes to POS and the White Cable goes to GRD.
Those may not be your actual battery cable colors but that is the way the cables should be at the battery terminals.
RED cable on one battery POS post and WHITE cable on other battery NEG post
RED cable coming on to converter
WHITE cable coming into converter and going to frame ground
12V DC items in coach are working cause when plugged into AC Power the converter is supplying the DC System but can NT charge batteries due to reversed polarity condition
