[Carl n Susan]

The black tank drain should be pushed in (closed) when parked. You only want to open it when you dump. That way you achive a good hydraulic flow effect to get all the solids out. If you leave the valve open when parked, the liquids drain out but the solids don't. This results in the dreaded pyramid of poop and dried out mound that will block furture use.

The grey tanks (you do know there are two?) can be left open (particularly if you have a washer/dryer that drains to the shower tank) or closed (dump them before they get too full, ideally when you dump the black tank to help clean the drain line). If you leave them open (which we do), be sure to build a "trap" in the line to avoid a clear path for sewer gasses to return to the RV.

And one last point, If you weren't aware of the two grey tanks (shower and galley), then perhaps you don't know there are two separate pull handles. One in the convenience center which is easily found. The other is under the RV on the drivers side, somewhere near the wheels In front, between, or aft) depending on the model.