Even if you’re not in the camper, it’s recommended to keep the auxiliary heater on.
8-10°C (46-50°F) is sufficient!
This way, you prevent the tanks and pipes from freezing, which can cause costly damage.
Be aware that the heater uses not only diesel but also a fair amount of electricity. Typically, you can be self-sufficient with your Roadsurfer for 1-2 days before you need to recharge the battery by driving or plugging into shore power. Each evening, check that the camping battery has at least 12.3 V (50%) – otherwise, it’ll be a cold night! If the heater doesn’t have enough voltage and you try to start it several times unsuccessfully, the system may lock itself completely for safety reasons.
Generally, you’re safer on a campsite with shore power in winter, as this prevents heater errors due to low voltage.
You can find general information about winter camping here.