You may Not want to add a washer/dryer for several reasons, just give it some thought.
Not only does it take up valuable space, but it also introduces lots of moisture in the limited space that can cause a lot of problems during the cold winter months.
Even though it may be less convenient, it is usually more practical to use the laundry facilities within the park or local Laundromat.
Also most people always leave their water and sewer hoses connected up as you would have to with a washer/dryer. As mentioned earlier, you would have to deal with heated hoses during the cold months.
What I do (w/o washer/dryer) is function from the internal tanks that don't see freezing temperatures.
During a day off, hopefully when the sun is shinning, I dump the holding tanks, refill the water tank, and then put the hoses away. That way there are no hoses to freeze up. Also doing this in the sunny months the hoses last much longer while not always out in the UV rays.