Caravan wheels are pretty standard, very few different sizes. I'm guessing that age it will be 13inch 4 stud, yes? If caravan has Al-ko chassis probably this one & if it has BPW chassis this one. You can often find them secondhand or caravan shops will stock new rims for £30odd.
Easiest way is to check pcd, distance between wheelstud centres across centre of wheel. 100mm or 5.5inches.