Sorry, Pierre, but Ambergris Caye doesn’t have an RV option. This is a fairly small island and you are only allowed to rent a golf cart here. Most expats get around on foot, on a bicycle, or with a golf cart. RVs are very unusual in Belize. I have only heard of a RV park outside of Corozal and near Benque Viejo, in the Cayo. So you would be better off to rent a 4 wheel drive vehicle and get around that way.
To get to the cayes you will fly on a small plane, or take a water taxi.