I tried to drain my FW tank and ran into the same problem. There may be a one way check valve in the tank between that line. To work around this I ran my faucet until the FW tank was empty then filled it with water and a small amount of bleach to disinfect. Be careful to watch the water pump so is does not run dry and burn up. Just run it until you begin to hear cavitation from the pump.
'98 Gulf Stream Sunsport 325, 7.5L Banks Power Pack, Koni FSD's, Air Bags, ReadyBrute Elite,
2000 Honda Accord