The solar input of the BCDC1240D requires unregulated panels, so a regulated panel will not work. If you are unable to bypass that regulator, you would need to connect the reg direct to the battery. I.e. you must have ONE reg between the battery and the panel. Using TWO regs, (the panel reg and the BCDC reg) will not charge.
If you drive for 4 hours with the BCDC1240 charging at 40A, that is only 160AH into your 320AH battery bank (and it will probably only store 130AH of that), so depending on how flat your batteries were when you started, you may not reach full charge (keeping in mind that once past 80% or so, the current being accepted by the battery will be tapering off, meaning it can take even longer to reach 100%)
Regarding the rest of your enquiry, I would need some clarification. As enquiries such as this may require a lot of back & forth emails, to find an accurate & timely solution, I recommend you call REDARC Tech Support on 1300 REDARC (1300 733272) when you are at the unit with access to the battery and associated wiring and with a multimeter in hand.