The Keystone website
http://www.keystonerv.com/previous-y...ugar&year=2009 shows that the 320SRX (both east and west coast models) have 60 gallons of fresh water, 38 gallon black tank and 76 gallons of gray water capacity. That would "add up to two gray tanks of 38 gallons each. Typically they are connected one tank to the bathroom sink/shower and one tank to the kitchen sink.
That being said, it sounds like this trailer is "new to you" (based on your not being familiar with the tank sizes) so there's a possibility that a previous owner had problems with one of the gray tanks and replumbed the trailer to eliminate the "bad tank".
So, even though the trailer model "calls for" two tanks in the specs, there is a possibility that your trailer may have been modified if you're not the original owner. You can do some looking under your trailer by pulling down an edge of the coroplast liner and using a flash light to peek inside to see what's actually there.
Good Luck !!!