£12 for cleaning the windows on a mobile home is fine, there's generally only about 7 of them, if you're cleaning the whole caravan then £12 is a ridiculous price, we clean them for a caravan park every week and even with bulk discount we still charge £25.
Also forget doing it using wfp, get some traffic film remover in a bucket and wash the whole caravan off with that using a long car wash brush, then plump into the mains water supply and hose the whole lot off, scrim the windows. 30 mins max