from reading this thread I'd believe the tanks are plumed wrong for sure,
if you dont have underbelly easy to see, simply look at plumbing drop from shower, should go to larger tank (grey) drop from Kitchen should to go galley(grey smaller tank)
since you have one outlet both tanks should drain into that line individually with separate valves.
If you do have underbelly it will have to be removed to access problem,
if in fact this is the case the only resolution is to replumb the whole thing, draining the shower and bath sink into the large tank, the kitchen sink into the small tank.
and FYI you dont want to be caught draining any water onto the ground in most camping areas( even if it is from the kitchen) big fines and or removal from camp ground,
Regards, best of luck.
2006 Expedition 4x4 w/HD tow package
5.4L 4 speed auto w/OD 3.73
tow rated 8200
2008 Cougar XLite 30FKS