why separate tanks for grey and black water?

Keep in mind that two tanks give some options for weight distribution too.

I agree with all the posters here. Now on occasion I have equalized my tanks when I need more grey water room by opening the grey valve and leaving the black tank valve closed. This brings them to the same level. So at them time its like I have only one combined tank. But that is when I am dry camping. My TC has 46 gallon fresh tank and I know I can run it dry with the capacity of both tanks being used. But usually I keep them separate.

BB_TX wrote:
There has been a couple occasions when I forgot to check the grey tank level. Backed up into the shower while I was in there. Found myself standing in an inch or two of water. Glad it was grey and not a mix of black.

X2

There has been a couple occasions when I forgot to check the grey tank level. Backed up into the shower while I was in there. Found myself standing in an inch or two of water. Glad it was grey and not a mix of black.

You can combine them and that new tank will be called the "black tank". Jurisdictions may have different dumping parameters for the different types of tanks.

none other than practical.
gray water can be dumped, sometime, at some places, on the ground, legally, where black water must always be dumped in a legal sewer receptacle.
so sometimes, you can extend your stay at a campground without moving your rig.

Thats what I did. It worked fine.