How can I fix small holes that are appearing in the outside caravan Walls. ?

I've noticed a number of holes appearing in the outside covering of my trophy caravan. Can I fill these with a sealant to stop water getting in? They're only millimetres in size but lots of them. Apart from this the van is in good condition. It's not worth much as it's 20 years old but it's too good to scrap. A cheap repair would be appreciated.