No laws or regulations that I know of. When you dump your tanks, black and grey, don't they go down the same sewer pipe anyway?
I think multiple holding tanks are added to RVs to increase the capacity they can be used. You can go a lot longer before having to dump with a 50 gallon thank than you can a 25 gallon tank.
Another reason for multiple tanks, (perhaps) is because manufacturers could use smaller tanks, but more of them. Dedicate one for the toilet, one for shower, one for the kitchen sink and they don't have to be as big. But the over all holding capacity is the same. (for example, you can have one 75 gallon tank for everything. At capacity that's 75 gallons multiplied by 8 pounds per gallon equals 600 pounds. If you had 3 twenty-five gallons, the weight of each one would potentially be 200 pounds.) And their overall size is smaller, easier to tuck away into the frame.
Other than convenience, size, and weight, there's no other reason why there's a difference between your black and black tanks separate or combined into one.
