Our Heartland Trail Runner has two sewer outlets . . . one at the rear for the kitchen and one in the middle of the trailer that handles the shower gray tank and the toilet (black) tank.
I've noticed that the dual-use one in the middle of the trailer drains slower than the one at the rear that only is hooked up to the one (kitchen sink) tank, but it still drains in a reasonable manor, so it doesn't throw up a red flag for me.