One way to check any tank appox. capacity is to take a garden hose and fill a 5 gallon bucket while watching your watch as to how long it takes to fill the bucket.
Then run the hose into the coach and fill your tank in question while watching how long it takes to fill the tank completely, not by the level gauge, but actually is full enough to fill to the drain so that it won't go down. Watch the shower/tub drain as it will back up there before a sink drain will.
Take this time, divide by the time it took to fill the 5 gallon bucket, then multiply times 5 = the amount of gallon size of your tank.
This method should be close to the size of your tank in your coach.
If you don't want to do this, then fill a 5 gallon bucket and dump down your drain and repeat this until tank is full. Count the amount of buckets and multiply times 5 to get your tank capacity.
Oh, BTW I believe all RV water heaters are either 6 gallon or 10 gallon in size. These are the size tanks that Gulf Stream uses. Also some manufacturers include the hot water tank size in the figures for the fresh water capacity. I am not sure if Gulf Stream does this or not but according to my test figures on my coach they do not.
Many storage tanks in our coaches are made with "steps" and are not the same size all the way across, I know my fresh water storage tank is. It is made to go almost all the way to the top of the compartment on the passenger side but it is made to slide under the frame rail on the drivers side. It is 90 gallon in size, believe me, I checked.
Also, the black and gray are 75 gallon each, once again I checked them with the above hose method. The figures came out within just a couple of gallons each.
Ron