I understand the problem now and believe that the NASA Battery Monitor (or most other battery monitors) should do what you want.
You can wire the shunt in the feed from the solar panel – I think it’s the negative feed recommended but it shouldn’t actually matter as you are only interested in the volts drop across the shunt (and can reverse the sensing wires or mentally reverse plus and minus in the readout!). The power to drive the monitor instrument can come from the service battery as per the recommended wiring diagram.
You can reset the amp hours readout to zero when required and it will then give the amp hours total delivered by the solar panel over 24 hours or whatever time you want before you do the next reset.
Regarding the grey instrument panel, I would like to replace mine as the original looks a bit ‘naff’; however, it is still in one piece so have not quite had the incentive to pursue a solution. Later models (including the 40 Ocean) have a wooden panel. It might be worth asking if Bavaria could either provide the bits to build the wooden version or possibly provide the drawings. It will be a good challenge, though, to fit the panel and all the instruments and switchgear again! Good luck (and please let me know how you get on).