We have a 2016 34RSBS and asked the same question. I was able to get the plumbing schematic from Jayco and the front gray tank is there to support the washer/dryer connection. We solved the problem by adding a twist-on valve onto the sewer hose connection allowing us to use both tanks together. When you hook up, you put the valve on, then open both gray valves (make sure the black is closed) and they'll seek level using the connecting piping. At dump time, you have to close the gray valves, then open the twist-on valve to drain all the connecting piping, then open the black. I drain each gray tank separately after that. Here's a link to the valve so you can see it:
Twist-on Valve - Valterra T58 - Sewer Fittings & Adapters - Camping World
Good luck.
TxHoo83
2016 34RSBS
2007 Chevrolet 2500HD Duramax