The size of the tank is also determined by the model/location of the bath, etc. Sometimes the black and grey are swapped. The online specs for each model will list the size of each tank. We frequently equalize as stated above by opening the valves as our black fills far slower than our grey. I also do dishes in a Tub-Trug and dump that water down the toilet so it goes into the black tank. If you do this, be VERY careful and make sure you don't dump utensils or other non-macerator friendly items. I like to dump into the toilet THEN open the blade to allow the water to go down. Just one extra chance to catch something I missed and it adds a little soap to the toilet bowl. Our 2552 with rear bath has a smaller grey water and larger black water tank. If we are on extreme water restriction, we also do bucket baths and that water goes down the toilet and into the larger black tank. We haven't camped anywhere we could let the grey water trickle. That would be a nice solution but so far we have had to keep everything in our tanks.