Since it's a 2013, I'm guessing it's a 3600. I don't believe Voltage came out with the V3605 until 2014. According to the brouchure, it states you have:
Fresh: 160 gal.
Black: 96 gal.
Gray: 96 gal.
Now here's where things get a bit confusing, you have one fresh water tank, two black tanks and two gray tanks. The main bath has the Black/Gray dump handles in the basement convenience center. Under the main slide, you have another dump valve that you must crawl to reach which is for the kitchen sink. This gray galley tank empties into the same sewer outlet as the main bath black/gray tanks. For the half bath in the garage, there is a completely separate tank and sewer outlet behind the axles that you have to connect to the sewer hose in order to dump. It's because of this I don't use the half bath at all.
So the way I have it figured on my rv, you have your 160 gallons of fresh water, a 48 gallon gray and black tanks in the front, then a 48 gallon gray for the galley, and finally a completely separate system consisting of a 48 gallon black tank and sewer outlet for the rear bath.
I have attached the 2013 brochure for you to see for yourself. A lot of info, but none of it very specific.
2013 Voltage Epic 3600
2006 GMC Sierra 2500HD Diesel (LBZ)