It's not necessary to have a full grey water tank to flush the lines after dumping the black tank, several gallons will do. Then be sure to put a few gallons of fresh water in the black tank after DW drains it. Never want to leave it empty. You may or may not want to add some soap or toilet chemical then, too.
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'11 Eagle 320RLDS
'02 F350 PSD Dually 4WD
DW's Ride, '13 Expedition
'14 Denali XL
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