If both are draining slow, then maybe the same cause. Perhaps a clogged tank (roof) vent.
Also on my coach, there are "auto vents" one at sink and one at shower, that allow air to enter drain pipe for faster drainage (and to keep p-trap from siphoning dry). If the vent was stuck open you could get grey tank odor and if stuck closed, you could have slow draining. So check those too.