175r14 has the standard aspect ratio of 80 (height to width ratio), very little difference to 75, so go ahead and fit the 175/80 R14s.
It will have no effect on the caravan.
If fitted to a car, the speedo reading would be very slightly out.
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My advice would be to contact your caravan insurance first and explain to them that the required tyre size as per handbook are no longer available and that the nearest tyre size available is 175 80 R14. Theoretically it is a modification but totally out of your control so if you ever do have to make a claim in the future then there is no get out clause on behalf of the insurance company due to the wrong size tyres being fitted.
If you search eBay you will find the size you mention but they are only 86 load rating which for 2 tyres give a combined rating of 1060kg. If the loaded weight of your caravan is more than that then 86 load rating is insufficient.
I would suggest you fit 175C80x14 tyres. Only half inch higher & have a 98 load rating total 1500kg for 2 tyres which should be sufficient for caravan.