We had a similar problem with our new Rockwood with our sink draining too slow through a piece of garden hose (using the gray adapter fitting supplied by Rockwood). Note: We don't have a shower on our A-Frame.
I surfed the web (kind of a common problem with a number of similar solutions) and built my own drain that connects to the side of the camper and replaced the stock adapter setup with plastic sink drain parts. I used a threaded "T" with a nut (and a couple of other parts) to connect to the side of the camper. I capped the top and drilled a couple of small "vent" holes in it. The original fitting screws into the bottom of the "T". The hose attaches to the fitting and drains to our gray water container.
The sink now drains properly. (Cost was less than $6.00). See the high quality diagram (below).
| <-- The side of the camper
|...... ,Capped but vented at top
|]= <-- Sink "T" (attaches to sink drain on side of trailer)
|.....\\ <--Drain adapter at bottom connected to hose going to gray water container.
(Imagine a beautiful image - just remove the dots (periods)
I hope this is helpful. If you want a real picture let me know.