I sometimes use a week hypochlorite mix 1part hypo to 15 parts water.
but at the moment, i tend to use No Nonsense degreaser from screw fix. I find it excellent.
use fan spray jets to thoroughly soak the gutters etcwith the chemical. go back to the start, gentle rub and rinse
I have one to do today, and 2 caravans. if I remember I will take some pics