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.
1975 American Clipper RV with Dodge 360 (photo in profile)
1998 American Clipper Fold n Roll Folding Trailer
Both born in Morgan Hill, CA to Irv Perch (Daddy of the Aristocrat trailers)