We have the 2016 Eagle 314 BHDS. We have dual grey tanks. I can tell you for certain, the only thing that drains into the rear grey tank on ours is the outdoor kitchen sink. I ran all of the systems with clean water before first use to make sure what everything did. I did not want any surprises!
2016 314 BHDS Eagle
2016 FORD F250 SD LARIAT CREW C
6.2L V8 OHV 16V
ProPride 3P Hitch - 1400