Once home and you've got the majority of it off try some Oxalic Acid, mixed with wallpaper paste (helps it stick to do its thing). 10 mins with that on and the hull will be like new. Just remember to clean it off fully, including the trailer as it will burn away the galvanising.
What sort of mix is recommended and should you begin dissolving the Oxalic Acid crystals before adding the wallpaper paste or both together? Elsewhere, I've seen a suggestion of 50g per litre in warm water then add 1.5 tablespoons of the paste when cooled. Also, how best to apply it and how much coverage do you get? Goggles and gloves seem to be thoroughly recommended!