There are several ways that I do it and we hardly ever use sites for overnight.
1. Many campsites will let you empty the loo and fill up with water for a fee, the camping and caravan club sites for example charge £7.50 for 3 hours on site, which includes using the showers, should you so wish.
Many private sites are ok with it too, for a fee.
2. Club Motorhome stopovers. I have emptied the loo and filled up with water at many, just by asking if I can. Many don't actively promote it but are ok when asked.
3. There are a few 'official' aire type facilities that provide the continental style facilities, such as West Beach Hayling island and Canterbury Park and ride.
4. Garages when filing up with fuel are often ok for filling up with water.
5. Boatyards and marinas are often ok with loo and water and many are ok for an overnight stay on the quiet too.
6. Shops and cafes in tourist areas, after your custom are often ok with loo and water .
Probably the most controversial.
7. Public toilets, if no other option is possible, tip the cassette down the toilet slowly to avoid making a mess and clean up afterwards if you do so.
I also have a 5 litre can for water with a length of hose attached, so I can connect it to the basin tap and fill it up.
Also you will be surprised the amount of public loo's that have a tap round the back of the building, if you look.
Discretion is the better part of valour in this case!
Basically there is no nationwide 'system' for doing it, so you have to be cheeky and ask in a friendly way and expect a no response.
Its surprising how positive people are most of the time, for example last summer we did over 3 weeks Cornwall, Devon and Somerset, didn't stay on a site overnight at all and I used 3 Club motorhome pubs, 1 private campsite and 1 public loo for my services, all were happy to help. Well the public loo didn't have much say!
Grey water usually drips discreetly on the grass when parked up or on my veggie patch at home, keeps the insects off!
Is there anywhere in particular you are aiming to visit here?
Rolling on and enjoying life on the road