After a lot of searching a suitable DC/DC converter was located at a reasonable price. Meanwell make a wide range of power supplies including the RSP-1500-12. This power supply will operate on 127-370VDC and supply up to 125A at 12VDC, perfect noting the additional electrical loads for the power steering pump and the vacuum pump. The output voltage can be adjusted up to 13.5VDC to maintain the auxiliary battery at full charge.
The water heater for the cabin also arrived. Unfortunately the thermostat was damaged in transit. It looks simple to replace and, as it was sent from the US, hopefully the supplier can provide a replacement thermostat under warranty that they would be happy for me to install.
Two under tray pannier boxes also arrived. One of these will house the power cord and the other is for balance. I plan to fit a rotary limit switch to the door to disable the motor controller if the power cord is not secured in the box.