There are two marinas in St Raphael, one small (Vieux Port) and one large (Santa Lucia). Are you sure you checked the large one? I delivered a boat there this last winter and there was no problem getting a 4 month contract for that boat. Otherwise, you could also check Hyeres, which is probably full in the water but might have space on the hard. Thirdly, you can usually find space in St. Tropez during the winter if you are just sure to be gone by April 1st. You just have to wait until earliest mid October to arrive. First weeks of October is the St. Tropez Regatta and the harbour will be full.