I am guessing since both the models you mention are front bedroom they have one grey tank and as such one place to hook up to dump both black and grey.
My 3008W has the front kitchen so we have a grey tank up front with its own dump valve and then another grey in back for the bathroom that is dumped the same place as the black. We have only taken it for on one long trip and the kitchen grey only required being dumped once, when we were leaving. Having two grey tanks does double the grey capacity.
~Mike & Kim~
2014 Rockwood Windjammer 3008W Diamond Package
2012 Toyota Tundra Doublecab 5.7 4WD