If the tyres say 175R13 on the sidewall this size is now often indicated as 175/80R13 so that is your replacement size. You would be better to have commercial van tyres for that caravan to give you adequate load rating.
So the size you want is 175/80R13C. Although some makes will still be indicated as 175R/13C this one for example.
------------- Regards, Jack+Jon.
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I'd recommend having a look at Event Tyres. I've used these several times and found them to be the most competitive, plus they will come and fit the tyres wherever you like - driveway, van storage etc.
They currently show a Maxxis trailer spec tyre, 175/80R13 97N UE168 for about £68 all in.