RedRover,
RV Owner's Manuals are not typically model/brand specific, they're rather generic & cover basic information.
From the Cougar specs page: 38 gal tanks are a common size among the current Cougar line, [313RLI black tank is
also 38 gal], so chances are your 2 gray tanks are also 38 gal each.
http://www.keystonerv.com/cougar/#/specs
Keystone Customer Service
http://www.keystonerv.com/customer-service
The inconvenient (& separate) galley tank valve isn't exclusive to the 313RLI... it's inconveniently located on
other models & brands too.
Similar location on our previous 5th wheel, so DH used a plastic-covered stadium
seat cushion to kneel on (wet ground). The plastic can be cleaned (ex: anti-bacterial wipes) after use.
Hope this helps,
Terri, the Chevy co-pilot