Eurochallenger -2008 statistics show 334,000 resident ( probably nearer 400,000 now) 4.4 million people visited creating 23 million visitor nights in Cornwall as a whole. A very large percentage ( around 60%) of these would be bringing a caravan or motorhome, camping, coached in or staying in holiday homes so will almost certainly arrive by road and will be mostly UK originating.
The fact 400,000 people are resident in Cornwall is misleading as far as Newquay Airport catchment area is concerned since many living East of Bodmin could be described more in the Plymouth or even Exeter catchment area.
Another factor which is relevant is that being mainly a tourist area many locals derive their income from Summer visitors so are far more likely to holiday outside of the tourist season.
It is impossible to give all the information here but hope that helps you a little.