Hi Lisa
We have to travel a similar distance to you, from St Helens, and have used several options over our 5 or so years of doing this:
(1) Overnight at Maidstone Services. We've done that twice. Okay but limited number of caravan pitches and could be full as yours is a busy weekend. However this would probably mean you arriving rather early at Dover.
(2) South Mimms Services on the M25 clockwise. You should just catch it if you are coming down the A1(M) Large carpar with a number of spaces for caravan. Used once once. You could time leaving here so that you wouldn't be too early for the boat
(3) Little Switzerland Campsite in Folkestone. Done 3 times. A favourite of ours with Channel Views to die for!! See here for details and reviews, including mine (Tim and Cary)
(4) Little Satmar campsite, Folkestone. Done once. We were put on a large grassy area so that we and other channel hoppers could stay hooked up. Details here:
http://www.ukcampsite.co.uk/sites/reviews.asp?revid=1538
The campsite option would mean that you could stay until 12 noon and then arrive at Dover in perfect time for the boat. If you arrive early parking to wait for your boat is a PROBLEM - nowhere in the Docks.
http://www.ukcampsite.co.uk/sites/reviews.asp?revid=1539
Thurrock Service, clockwise on the M25 would be another possibility just before the Dartford crossing. I've not stopped here however.
There are other campsites near to Dover too - If you do an area search on ukcampsite.co.uk you will be shown lots complete with user reviews.
Another thing we and you will need need to consider if you are travelling Clockwise on the M25 are the holdups at the Dartford crossing - see here
http://www.ukcampsite.co.uk/chatter/display_topic_threads.asp?ForumID=7&TopicID=197200&PagePosition=1&ThreadPage=3&ReturnPage=Search&search=&searchIn=
The last post on that thread warns of 8 mile/2 hour queues southward at the Dartford Crossing on Saturday 17 July!!! We may consider going anti-clockwise on the M25 if traffic reports at that time say it's better but that's not such a good option for someone travelling from Eastern side of the UK
I hope this helps. Questions, questions but worth it when you get there. Have a super holiday.
Tim