So long as the hole pitch diameter and the wheel offset is the same, there is no reason why alloy wheels cannot be fitted. However, if you want 14 inch instead of 13 inch wheels you will probably have to fit lower profile tyres so that the overall diameter remains more or less unchanged or else you will lose wheel arch clearance. However, lower profile tyres will result in a harsher ride and shake the contents of the caravan around a bit more.