Hopefully other owners will chime in, but it sounds like a mistake to me. However, since you say it's the rear grey tank (so you can still flush with the front gray tank) that may be why they did it that way.
In any case - be careful. If your black tank is fuller than that rear gray tank, and if you inadvertently open the gray valve, you'll be putting black tank water in the rear grey tank. Not good.
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1988 Coleman Sequoia - popup (1987-2009) - outlasted 3 Dodge Grand Caravans!
2012 Roo19 - hybrid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado 1500 Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10
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