Actually, do as "chuckftboy" suggested, but start with the water tank. Fill it completely with a hose. Drain it into a five gallon bucket and poor that into the grey tank until filled. Counting the buckets out and in. When you're done, poor the grey tank water into the black tank. When you're done, you'll have all your capacities and clean tanks.
Don & Mary
2019 Newmar Dutch Star 4018 (Freightliner)
2019 Ford Raptor