Originally Posted by groswald
Hooking the bathroom sink into the black tank seems to fairly common.
The bathroom sink in my 28Z is the same... plumbed into the black tank.
I can think of four possible reasons for this:
1. as you said, to extend the "capacity" of the grey tank.
2. to add a bit more water to the black tank for proper decomposition - as many people probably don't rinse the toilet long enough after use.
3. the plumbing route to the black water tank is closer/easier/less costly than to the grey tank.
4. a built-in rinse mechanism to put a clear water flush into the black tank without having to install a dedicated tank flush port into the tank.
For my rig - I'm guessing #3 or #4.
#4 at least is useful, yet may actually be an undocumented feature. Simply run a couple of gallons through the tank via the bathroom sink, and it will (hopefully) flush any residual TP, etc.
If done for this reason - it is a great idea by Thor - albeit they don't document it.