It is becoming more common for the larger RVs to have their showers tanked to the black holding tank. On many forums (RV) there are many members asking how to re-route their gray water to be shared with the black tank. Commonly, the gray tank will fill long before the black tank does; dishes, washing hands, cooking, grooming, etc. are things that will consume the largest amount of water - directly to the gray tank and aren't "optional"...they will happen daily. Showers are something that can be put off (alternate days, every three days, etc.); navy shower techniques can be used and for the boondockers; the outside showers can be used to "hose off" children (and adults) as interim measures along with "cat baths".
My personal experience... My black tank will last a week for the two of us (toilet only), while my gray tank will last three days and we don't use our shower (we rely on campground facilities). In any event, whether gray or black tank for the shower, you will have to learn to conserve water...not only for the usage from the FW tank, but, for the "storage" in the waste tanks. As opposed to a sticks and bricks home; an RV not only has a finite supply (unless hooked to shore water), but, also a finite holding capability. If you don't have a full hook-up site, whether the shower goes into the gray or the black, the tank that it does go to will fill up quickly unless conservation measures are taken. If you have full hook up provisions, then it doesn't really matter...whichever tank fills first can be partially drained until ready to leave. Another benefit to having the shower go to the black tank is better flushing of the contents. Far too often, the black tank doesn't have enough water in it to do a good job of flushing the solids from the black tank, leading to clogs, dirty sensors and odors. The additional water the shower puts in the tank leads to a more complete emptying of the black tank.
Your "situation" is not an error and is actually "sought after" by many long-time RVers (myself included). Conserve and enjoy your new trailer
