I've only been on one site where the grey waste point was raised, and that was an agricultural showground which occasionally hosted conferences where the delegates camped / caravanned. Just as Mandarin describes, the grey and black waste points were side by side and about 3 feet off the ground. I found an "unofficial" grey waste point behind some portacabins where a manhole cover had been left open for the grey and black waste from the cabins to be piped into (I was on site as a volunteer worker and it was an area I was allowed to be).
Everywhere else I've used, grey waste has been at ground level, except two sites where I was allowed to pipe my grey waste to a land drain alongside the field, without using a wastemaster.