Hi,
Your caravan window will either have been constructed from Acrylic or Polycarbonate plastic - and will probably have been ultrasonically welded together.
if your window is Acrylic (most likely), you can either use a solvent cement like WeldON #3 - try a fishtank shop - or a two part superglue. I don't have a reference for the two part superglue - but it needs to be a "low fume" type otherwise the windows can go "frosty"
If it's polycarbonate - then the same two part superglue is the only solution.
The glue join must be clean and grease free - Iso-Propyl Alcohol (IPA) is good.
Robert