Hi Miamoo, welcome to the forum and congratulations on buying your caravan.
Ask your friend how old the caravan tyres are, even with a good tread pattern they should be replaced between 5 and 7 years old because the sidewalks deteriorate and motorway blowouts become more likely.
Any proprietary automotive shampoo will clean the caravan, black streaks are usually caused by dirt and dust on the caravan roof which streaks the caravan when wet.
A neat application of shampoo and a gentle rub will remove them, don't be tempted to use abrasive products like T Cut because caravan paint is very thinly applied.
There were towing courses available from the caravan and motorhome club or the camping and caravanning club before Covid, these will probably resume when government restrictions are lifted, but alternatively you could practice manoeuvring on a large supermarket car park when they are closed.
Enjoy your caravan 😊