dear Blade, we use a fixed ball and always have, as for single/twin electrics,
single electrics just supplies the exterior light on your van
twin electrics will charge your battery and fridge BUT, it takes too long to charge the fridge, in fact if its not over a 3hr journey or in hot weather the fridge will not be very cold on arrival unless you plug the mains hook up in and get the fridge cold before you set off then the 12 volt twin electrics WILL keep it cold, we never use the grey plug(twin) anymore, but the other good thing with twin electrics is, if your battery goes flat you can hook the grey plug in and run your interior 12 volt supply for lights and so on from your car, but we have only ever needed to do that once