In the first place you would think ALKO's design department could come up with a spare wheel carrier that could be removed from the rear of the caravan,
I refer you back to one of my earlier comments, the chassis's supplied by Alko are not just destined for caravans but a wide range of other types of trailers, so in practice Alko will not know what form the back end of the trailer will be like, making it almost impossible to accommodate all designs. You only have to look at the details of different caravans to see there can be wide variation in the rear end design.
Some types of trailers are designed to be tipped so the rear end will touch the floor, so a spare wheel carrier there would be a major disadvantage.
I would be tempted to suggest it might be fitted under the front of the trailer, but my own point about what is fitted by the final trailer manufacture could prevent that , just as could the members that create the A frame.
Its easy to criticise but often much harder to come up with a practical solution.