Using the black water tank as a second grey water tank

I agree with the rest.

We always flood our black tank with gray water to flush it better. If we never used the toilet, I could easily see opening the right combination of valves so both tanks share the gray water you make.

As others mentioned, be sure to thoroughly clean out your black tank just in-case you later get a back-up into the shower pan. I clean our black tank annually with 1/3 gallon of bleach and fill with water to the toilet tube, let sit for an hour or two, drain and repeat twice without bleach to flush well. Bleach is hard on waste valves so you want to make sure you flushed it well.

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We had a TT with 2 40 gal. grey, 1 30 gal. black. Of course the shower tank filled before the galley or black.
To extend our stays I made up a 4th temporary valve placed on the dump, then opened the 3 tank valves which allowed equalization in them so the entire 110 gal. capacity was used.
Mixing the grey and black actually helps on dumping. The extra water in the black helped flush the solids.
Do it.

As others indicate, no reason not to do it. If the black and grey tank currently empty from the same opening, then you are already committed to emptying them to a sewer.

If the grey tank currently empties through a separate opening, then it gives you the option of emptying it and using the grey water for plant watering where legal. In that case, I'd keep them separate so any residue from the black tank doesn't prevent that ability as it can give you a bit extra tank use.

Before doing the repiping, I'd do a trial run with each group of faucets. Empty the grey tank and just run the shower every day as intended and see how quickly it fills the tank. Empty the tank and then do the same with just bathroom sink and then just the kitchen sink. Then you can split them out to best fit the tank sizes.

Flush it out a couple of times with bleach and use it.

As others have said, there is no apparent reason why you couldn't/shouldn't do this. There are likely several different ways to plumb it, that'll accomplish the job.

One thing that occurs to me, is you may want to plumb a connecting line between the tanks so they self-level. Otherwise, one tank may consistently fill up before the other. Being as how there is no solid waste, you wouldn't need a big line to do it -- 1" would probably be plenty.

And you might also think about making the mod reversible, in case you ever want to put it back the way it was, for resale.

If you do it, some pics and an article about the job would be interesting -- I'd enjoy following the work.

Could be as simple as adding a Valterra Black T58 Twist-On Waste Valve. By adding this, open both the black and gray installed gate valves, thus gray water will equalize in both tanks. No physical changes required on the existing tank plumbing.

I see absolutely no reason why you could not do that, regardless of how you plumb it. It's your camper, you can do with it whatever you want. And personally, I wouldn't worry about the next owner because that may never happen.

Considering black and grey tanks are dumped into the same sewer pipe at dump stations, there's nothing magical about keeping the black and grey separate in an RV either (not really). I think the idea of multiple tanks developed because it afforded smaller tanks, but more of them, which resulted in the end with more tank capacity. And if using smaller tanks, it makes logical sense to use tanks dedicated to one bathroom or kitchen feature. After all, they all come together at one outlet (valve) on most campers don't they?

All my travel trailers and now the 5er had separate black and grey tanks and pipes that outlet to one valve. Using a Flush King, I'd back-flush the grey water into the black tank every time I dumped to help clean out the black tank better.

There's nothing magical about the tanks. The only set-back I see is, it may take a while to clean out the black tank from it's original usage, because 'stuff' does splash around in there and even clings to the roof of the tank, which never get's "flushed" out and eventually dries hard as rock. If installing a shower, make sure you connect the black tank from the shower floor with a P-trap to keep odors from coming up, and make sure the tank is vented to the roof.

Go for it!