Hi have to park under trees were I work best and simplest solution found on a dull day not in sunshine mix a bowl of washing up liquid with warm water and gently with a sponge keep washing windows, over five mins sap will come away, make sure you rince well with water.
You can use chemicals ie tar remover but on caravan windows the chemicals contained can cause fine surface cracks to apear or glazing although they do not impede the water integrity of the window they dont look nice.
To help in future sounds daft but wax your windows with good car wax.
leave to dry and then polish as normal next time sap wont stick so strongly.
Hope this helps
John