Reads more like a charging problem.
Diesel engines have higher compression than gasoline engines so require more AMPs to start/turn over.
The dual batteries are connected in parallel so it is still a 12volt system, some use two batteries in series, so have 24volt starting system.
Anyway from your description of "slower" starting for a few days then a no start it reads more like the batteries were not be charged fully when engine was running.
I would do a check of the cables first, loose cables prevent charging and full battery power from getting to the starter.
Check Ground cables as well.
Then jump start engine and use a volt meter to see the voltage at each battery.
should be above 14volts just after starting, then it should drop to 13.4-13.7volts after a few minutes, both batteries should show the same.
If you see under 13.4 volts then batteries are not being charged.
Low or no charge could be Voltage regulator or alternator, if it is the origin regulator then just replace it.
After replacing then test voltage again and alternator.
If you have a "Battery" light or a voltage meter in the dash make sure it is working, the power passing thru either of these turns ON the alternator via the voltage regulator, so if "battery" light doesn't come on with the key, or voltage meter isn't moving when you turn on the key, then the wiring for these is bad, so alternator is not being turned on.