If you look up your model on the Keystone website, it should show your tank capacities. In our case with a 3791RD, it has 66 Gallons Fresh water, which includes the 12 gallon water heater and water lines making the tank itself around 50 gallons.
Black (waste water) is 49 gallons and the Gray water is 88 gallons, split between 2 44 gallon tanks, as I understand it. Look at your model here...
https://www.keystonerv.com/product/m...h-wheels/specs
The 3761FL and 3931FB both have 88 gallon waste water capacities and both have 2 bathrooms and 2 black tanks.