And if you do cap the drain, use a "male Adapter" then a screw on cap, in case someone wants to install the washer/dryer later.
Even if P Trap, over time the water will evaporate, so (assuming the washer/dryer plumbing), so just capping it will solve that problem.
We don't have stinky plumbing unless the rig sits for a while, then we have an order in the drains of each sink until we use it for a couple of hours and then the smell is gone. I put "Happy Camper" in my black and gray tanks to keep them clean. I use the "Happy Camper" for my black water treatment. Check it out.
Monty & Janet - 2007 Alpine APEX 40 MDTS
S/N - 75715 - Retired - Master Certified RV Tech