Last year we set the boat up with two thermostats, a normal one that controls the Eberspächer D4W diesel heater when we need it, initially the heating was just a switch on/off. I also fitted a frost thermostat both of these have a switch so they can be taken out of circuit.
So this year we made sure the diesel tank was fully topped up switch in the frost thermostat as low as I could above freezing (about 2°C).
Update Feb 2023 – This seems to have worked well again this year, we have used a few gallons of diesel, but the boat seems to be free of any frost damage.