To keep your WH on electric only you must resist the urge to
push the water heater switch to the on position in the control center
located above your broom closet.
The gray tank question is a trick question. You have 2 gray tanks,
The forward tank is where your washer, bathroom sink and shower dump.
The rear tank is where your galley dumps.
The correct answer to your question is; gray water goes into both gray tanks.
Hope this helps.
The recipe for perpetual ignorance is:
Be satisfied with your opinions
and content with your knowledge.