Battery Voltages vs Capacity
I''d encourage you to do two tests using Calder''s Boatowner''s Mech/Elec Manual. You''ll find both of them in the 12V section of the book (which I don''t have in front of me, and so can''t be sure of the terminology).
One is a 5-sec pull, using the house bank to start the engine but with the engine start feature disabled. This will require a bit of temporary rewiring. You''ll be viewing the voltage drop on the housebank (which, yes, I did notice is ''new''). Just how ''new'' and also healthy that house bank is will be reflected in the test.
The second test is measures the voltage drop across the circuit from battery bank to load (engine). The goal here is to look at things you might/might not be able to see, but which produce resistance in the circuit and therefore reduce battery performance even tho'' the battery(ies) may be potentially healthy. Corrosion, poor crimps and other gremlins will all show up; I''m sorry I don''t recall Caldern''s term for this second test.
I''ve used both these tests to diagnose battery and charging circuit performance and think they can be very helpful. Whether they reveal the particular problems in your set-up or not, they will tell you thinks about your 12V system that you''ll appreciate knowing.