We have just bought a 2nd hand Jayco Journey with dual 80L water tanks, but can't seem to work out how they work. Our Coast to Coast monitor said both tanks were empty, but when we tried to fill them, only one filler tap would allow water to fill. The other filler tap just kept overflowing as though it was full. Is it true that you have to manually flick a switch under the van to change tanks, as I read on another forum? Surely not! We couldn't believe we had used 160L of water for just 3 showers and 1 washup in 2 days!