Mine is not a hybrid, but we do use the shower ... always, with full hook-up or not. I have a blue tote which hauls away everything. Why have that space in your camper if you aren't going to use it?
Now, about water consumption? I CAN answer that quite accurately. How? Well, we driveway camp and we use the shower in the camper (even though we have one right in the house). So, I dump the gray water into a bucket and then dump the gray on our plants or in the field behind the house. (Yes, we live in the country where you can still pee in your front yard and no one is around to see you do it!)
Anyway, a few week-ends ago when camping we had full hook-ups so I used the opportunity to REALLY flush both black and gray tank. I came home with both tanks empty.
That night we drive-way camped and used the shower. Both my wife and I before going to work. That evening I dumped the gray into a 4 gallon bucket. The bucket was not even half full! REALLY! Both of us took a shower and between the 2 of us, we used less that 2 gallons.
When taking a shower, it seems like we're using a lot of water, but I suppose that shower head just makes it seem like we're using a lot. But the proof was in the bucket!
We Navy shower ... get wet, turn the water off, soap up, then rinse off.