I found on-baord tanks a mixed blessing - difficult to clean and fill sometimes. For my last van, I "made" one using a 25litre plastic jerrycan in the front locker with a second pump connection unit (Truma Carver sell them as winterising units). This could be removed for cleaning, and was easy to fill using 8litre ex mineral water containers. Despite having no insulation, we never had any prorlems with freezing in very cold weather.