caravan roof

I recently saw a roof box that is made for you caravans rather than cars, and I'm just wondering how it would work. As far as I've been told, it's not safe to stand on the roof of your caravan as the added weight can be dangerous on your roof. Considering this, would it be safe to use a roof box...

How do you clean the roof of your caravan? Do you use any specific products or cleaning tools? Also, how do you reach the top of your caravan? I'm not sure if this seems obvious but is it safe to stand on the roof of your caravan to clean it? It seems much easier than having to stand on a ladder.