The tank doesn't have to be completely taken off. It is possible to access the rad fill neck by moving the tank backwards until it is revealed. Raise the seat, disconnect the fuel tank connector, lift the rear free of the rubber tank mounts and pull the tank backwards. The length of the fuel lines may limit the distance you can move it rearwards, but if you tip it to the left once free of the front bumpers, it should offer enough room to access the neck with a funnel.