A lay by overlooking Dover harbour makes an ideal place to stay whilst waiting for the ferry. It is located on Upper Road abover the town. There are several lay bys on the A2 but due to the noise of the traffic it is difficult to get any rest. This lay by can also be a little noisy but it cannot be compared to the main road and also it offers fantastic views!
You may also park your motorhome on Marine Parade for up to 12 hours which is free after 18:00 and may be suitable to wait for an early morning ferry. My experience was that I had drunks banging on the side of the vehicle during the night so I won't bother again.