According to the Keystone website,
https://www.keystonerv.com/fifth-whe...s-fifth-wheel/ you've apparently got the black (waste water) tank capacity and the gray water tank capacity switched in the specs you posted. I'm not sure where you got your information, but this is what Keystone has published.
Key specifications - 308BHDS FIFTH WHEEL
Shipping Weight 9031 lbs.
Carrying Capacity 2669 lbs.
Hitch 1820 lbs.
Length 35' 6"
Height 12' 5"
Fresh Water 43 gal
Waste Water 78 gal BLACK TANK CAPACITY (2 tanks)
Gray Water 39 gal (1 tank)
LPG 60 (2 30 pound tanks)
Tire-Size ST225/75R15E
Sleeping Capacity 4
Region All
Looking at the floorplan, I'd say you have a 39 gallon black tank in the front bathroom that also collects the sink water and a 39 gallon black tank in the rear bathroom. The gray tank (also 39 gallons) is likely located under the galley and collects the galley sink, the rear bath sink and the tub/shower. It doesn't take much to fill 39 gallons when you're washing dishes, hands and showering.