Fresh water holding tank
The fresh water capacity is 55gal. Sorry, but can't remember the capacity of grey and black, but it is less than that. Just looked in my owner's manual: Main holding tank side bath 30, main holding tank rear bath 20. Auxiliary holding tank side bath 35, aux rear bath 20.
I sure wouldn't have guessed there was that difference.
Now which is grey and which is black is something you'll have to find out from someone else. Smile. Owners' manual doesn't say.