Hi SallySwift, welcome to the forum and to the world of touring caravans
Black marks or streaks are not uncommon on caravans so please don't do anything drastic like using an abrasive 'polish' on your caravan at this stage.
Black streaks and marks often appear on even the cleanest caravans because although the owner will have carefully washed and polished the front, back and sides the roof is often neglected. Most caravan roofs nowadays are flat and dust, oily deposits and grime collects so that after a rainy day the once pristine caravan is covered in black marks as the accumulated grime washes down the vertical surfaces.
The first step is to use a gentle hosepipe (not a jet washer) to remove the loose dirt from the roof and the vertical surfaces, step ladders may be needed.
Use a good caravan shampoo such as Fenwicks (
Click Here to see Fenwicks products) and an extendable car wash hose brush to loosen the remaining dirt, not forgetting the rubber trim along the roof edges and the awning rails.
For the front back and sides Fenwicks recommend starting from the bottom up, wet the surface, apply your diluted cap-full or two of Fenwicks ( a small garden spray bottle from the pound shop or Wilkinsons is good for this), agitate the shampoo with your brush, leave for a couple of minutes and rinse. If there are any remaining black marks use some neat Fenwicks on a cloth to wipe them off then rinse again
You can then polish the caravan with a good polish such as
Autoglym Super Resin Polish which will really clean the paintwork without it being too abrasive and if you feel energetic go over the polished surface with a good quality car wax containing carnauba such as
Meguiars Step 3
which will give lasting protection for about three months so that next time your caravan will only need a quick wash down.
The cleaning products are available at Halfords or good caravan accessory shops