I don't think I would worry about it. If the tank was full, and it was leaking, then I'd worry, but if it's just residual waste water left in the pipe which is what it sounds like you most likely have nothing to worry about.
Model X19H Expandable
GMC Sierra Pick up
Been a camper since I was a kid, and that was a long time ago :)
From pup tents with no floors, to tents with floors and flies, to a pop up, to the expandable.