I think the tank size theory varies among manufactures and even within the models. Most of the Monaco’s I’ve looked at, black and gray total the fresh. However on the larger Tiffin models, they carry 100 gal fresh, 100 gal gray and 55 gal black. Or in the luxury coaches, they just have fresh and waste. No distinction between black and gray.
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2006 Gulf Stream Friendship G7
350 Cat, Crusier Lift w/ Honda VTX
Blue Ox and 2019 Chevy Blazer RS AWD V6
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