We stayed there a couple of weeks ago. To be pedantically correct - yes there are signs saying cars and motorcycles only - but only along part of the Parade. There is also a section of the road with signs saying cars and caravans, so I think all is well! We parked in the cars and caravan part, but there were others in the car and motorcycle section, and the plods who cruised by didn't seem to mind. My guess is that even in this section, you'd probably be OK as long as you weren't there during the daytime, when the payment restrictions apply. As an aside, has anybody parked up overnight at the services at junction 12 M20 -those designated "Service and Ferry Early Arrivals". We didn't, but had a drive through. Several M/H's there but no area specified for them (nor caravans) - just for lorries and coaches. You ring a number, and pay an overnight parking fee. About 10 minutes from the port. Might be useful if Marine Parade fills up
Rick