[JRTJH]

I think the first objective, in this situation, would be to confirm where each drain collects and which tanks are involved with each drain. At this point, it's "suspicion" that the shower "might" drain into the galley gray tank.

I'd suggest emptying all the tanks, run the shower for about 10 minutes (about 2.5 gal/min = 25 gallons) and then walking outside, pulling the galley handle and "visually observe" whether the galley gray tank remains empty. Then do the same (starting with empty tanks again) with the bathroom vanity. Determine where it drains, then do the same with the galley sink, determine where it drains. If you have any other drains that collect in the holding tanks, then also test their "final drain collection point"....

To end this, without knowing exactly where each tank drains, you'll be hard pressed to mold or change behavior. As an example, at home, my DW runs hot water in the sink when washing dishes and rinses each dish in that "everflowing stream". She completely changes her dishwashing behavior when in the trailer and partially fills one sink to rinse soapy water from the dishes after washing. The first couple of times we camped, she used all the water in the trailer the first day, just washing dishes after lunch and after dinner. There was no water to flush the toilet that evening. It was all in the galley tank. Now, we can go for 5 days on our 43 gallon fresh water supply. That includes 2 or 3 showers each as well as dishes for meals where we use "other than paper/plastic plates".....

First, determine where each drain goes, then set about to modify behavior to minimize water use in those areas where the tank fills too quickly. Sometimes it's not comfortable to modify, sometimes it's easy. Every camper learns what to endure and what to change......