Hi Matt:
First of all, you do NOT have 2 "Gray Tanks" in your rig. (unless you had it special ordered?)
You have 1 Gray (for Bath,sink,shower,Dishes water).
The other is 1 Black (for toilet - # 1 and 2).
They ARE NOT connected together! They only time is when you dump them. You open the Black water valve FIRST. The follow opening the "GRAY" water 2nd.
2012 Concord 300.......
Number Of Fresh Water Holding Tanks 1
Total Fresh Water Tank Capacity (gal/l) 43 / 162.8
Number Of Gray Water Holding Tanks 1
Total Gray Water Tank Capacity (gal/l) 53 / 200.6
Number Of Black Water Holding Tanks 1
Total Black Water Tank Capacity (gal/l) 28 / 106
Better to have more then you need then to not have enough!