Hi n,
Welcome to the wonderful world of caravanning. There may be many things for you to worry about in the early days of owning and using your new van, when you get it, but emptying the cassette toilette will not be one of them.
Almost every one of the caravans made in the last 20 or so years has been fitted by the manufacturer with a cassette toilette. So the caravan sites of all quality, shapes and sizes are geared up for providing you with a facility for emptying one. At the top end of the scale the site will have purpose built "chemical disposal closets"; in more middle range sites it might turn out to be a standard toilette set in the ground with a screen round it; and even in pretty basic sites there will be a manhole cover that you can lift up and empty the cassette straight into the foul drainage system.
The site reception office will let you know where the disposal point is.
There may be a very, very few sites, perhaps in areas of outstanding natural beauty (where development and land use is extremely restricted), that do not have any facility - but they will clearly advertise the fact and you would need to limit your time on site accordingly.
At the worst, perhaps if you were staying on a friend's bit of spare ground for a short period, you can, with great care, empty the cassette into the house toilette.
Happy caravanning,
Mike A