I understood the problem to be the standard fit from MMC was a little wimpy and unable to power any auxilliaries let alone an inverter with subsequent losses.
The dual systems were more than adequate for the job as you intimate.
As in all things Morgan a uniquely

answer will emerge in the fullness of time and will probably involve stone hot water bottles - I don't feel the cold at all so "12Volt pants" are not on my Christmas list for wintery drives over Dartmoor

.