I have been to FoS twice and stayed off site both times.
The first time I stayed at a C&CC site nearby (Slindon), but that had no toilets and as it is mainly portaloos at Goodwood, I would prefer full loos for the evening.
We stayed at Bognor CC site last year and while it was further, it had full loos and a shower block!!
If you are staying off site I would recommend staying west of Goodwood as the Chichester bypass becomes a nightmare and to the west there are a few lanes so you can avoid the A roads.
Alan