The Camber can get busy, but as the free trailer park has been extended this year (compared to last year) there is normally enough parking and trailer space. If you phone the Camber office beforehand on 023 9283 3166 I or one of the Dry Stack guys could tell you the current situation (especially with Cowes Week looming).
As posted here before, there are sometimes (but not that often) fishing boats laid up on the slip for repairs, but we normally try to ensure there is still enough room for the public to launch.
Hope that's of use.
PS barrier to slipway costs £2.50 (you have to lift it, dont' expect it to open automatically or you'll be very disappointed)!!