Welcome to the forum and congrats on the new rig!
Here is a link to the specs for the trailer. You will need to scroll across to get you your model (this is the Keystone site and probably 2019 but I pulled up the specs from RV trader and they were identical from I could tell).
https://www.keystonerv.com/travel-tr...hideout/specs/
I think there is an error in the stats they have posted on several of the trailers. It shows the 28bhs with one gray tank with a 69 gal. capacity. I doubt that is correct. If you scroll across and just look at the specs for the tank sizes on the different models it appears they just sort of "jumbled" the numbers on some of them. In the case of your trailer I figure you have 2 39 gal. gray tanks. Many/most of them show that then others seem to show some sort of jumbled combo 39 (69), etc. I could be wrong but I'm virtually certain they didn't make a one off 69 gallon gray tank for the 28bhs.