agree with splashdown
a lot of time its down to how much gear you need so van/car is easy option
I couldn't go camping and just go with bike thease days
as for the empty spaces
serves them right I sussed out years ago don't panic book early on just keep looking far too many ferry spaces are reserved for the package companies trying to cash in on room/ferry combo deals
every so often they have to hand back somany unsold ferry places and then the racket puts them up for publice offering