I recently purchased a set (four) of Alloy rims for the car for when the road temperatures are warm enough that we can swap over to summer tyres - in 2-3 weeks hopefully.
There's nothing stamped on the rims themselves but my dealer, on their computer, have a list of which rims are suitable for which model of car within the range - I'm talking VW here by the way. More importantly, details such as allowable load are given against each rim and this should be at least the same, if not higher, than the permissible axle load stated in the vehicle registration documents/handbook.
I can't imagine that it would be much different for caravans. What's also important though, is that along with getting alloy rims one also gets suitable bolts for them. My Triton (delivered with Alloy rims) came with a set of extra bolts should I at sometime need to fit the spare steel wheel.