All caravans have two power plugs on the side of the van = an inlet and an outlet and they have different purposes. 

Your 15amp inlet is for plugging the power into your van. To power your van. 
If you are staying at a caravan park you plug a 15amp lead into this and then the other end goes into the power supply at the caravan park which is also 15amp. The caravan park has the correct end so you only need a normal male/female 15 lead. 

The other plug on the caravan is a 10-amp outlet. 
This is not for powering the van. This is for using power from your van. For example, you may plug your coffee machine or toaster into this outlet from your caravan.  If you are staying at a house that does not have 15amp then what you need is an RCD unit. It is illegal to modify a lead so that it has two ends a 10amp and 15amp - so if you are staying at a house and need to plug your van into it, you need an adapter. 

You plug your 15amp lead into the 15amp inlet on your caravan and then the lead is plugged into the RCD unit and then the RCD unit into the house. 

This youtube video by Tripinavan explains the setup you need  very well https://youtu.be/tqsSbH8otb0

This video by amphibian also explains why you need this type of set up
https://youtu.be/XJE6c0FrJ-U