I can confirm that there is designated car/caravan parking on the seafront road in Dover town (literally on 'the prom'). Check the signs on the lamp posts as only certain sections have this facility, but it is very convenient, safe (well lit) and legal (and its FREE overnight!). I use this every time, to be close to the ferry for early morning sailings - you can watch the boats from the van!). You can access the seafront from either the A20 roundabout by the Hoverspeed (western) terminal, cross the swing bridge and go left onto the seafront, or turn off the westbound carriageway of the A20 in Dover (behind the flats) to cross to the seafront road. (Be aware that there is a new width restriction on the turning in at the eastern end, so avoid that!). The parking is clearly signed, on both sides of the road. Its quite quiet here too. Highly recommended; out of bed and two minutes to the check-in! Have a good trip.