I've not been on such a tour, so maybe this would be unworkable for some reason I don't know about, but another option would be a 500mm mirror lens. Combined with your existing 28-300 zoom and 2x teleconverter, that'll give you a good set of long focal lengths, albeit some with very small maximum apertures. Mirror lenses tend to be small, which should be a big plus in helping to keep your luggage to a minimum.