What time is your ferry?
When I've caught an early ferry, I've stayed with friends a long way out in the countryside about 30 miles (about 45 minutes) away. Probably for the same reasons as you - Dover is not a particularly safe place to leave a valuable boat out on the streets overnight.
If you don't need to be too close to Dover, I can highly recommend this B&B in the Kent countryside, which has plenty of space to park your boat
http://www.dadmans.co.uk.
Because of various problems in the Calais area, last year I changed my route and went with Stena Line overnight from Harwich to the Hook of Holland. It's possible to get from Cardigan to Harwich during the day in good time to catch the evening ferry.