Harwich Port overnight in caravan

We are travelling from the North West to pick up an early morning ferry from Harwich to Holland and would like to stay on the dock overnight in our caravan.  We do this at Portsmouth docks, park up, legs down and sleep for a few hours until somebody bangs on the door to say they are commencing loading!  Can anybody confirm please that this is possible at Harwich and also what is the earliest time of arrival in the evening?  Many thanks in advance for any help.