We have a 63 Overlander and the black tank has about a ten gallon capacity, with the toilet sitting directly (more or less) on the black tank. I plumbed our galley sink to drain into the black tank. It works very well, although I have to dump it more often. Grey tanks came about in the 1970's, as people became more environmentally concious, and didn't want to dump their bath water on the ground under the trailer. In 1964
, no one thought much about it, so no grey water holding tank was included.
If you want to include your kitchen wash water in your black tank, as I did, you can go here, and scroll to the bottom couple of posts in the thread:
If you want to hold the rest of your grey water, you can get one of those blue tote-a-long tanks to hold it, and transport it to a dump station. Unless you are a real he-man, I would not recommend getting anything larger than 20 gallons or so, as you may have to lift the tank when it is full, and 20 gallons of what you put in the tank will weigh over 150 pounds.