Have just been told by a planning officer that yes you do need planning permission to put a mobile home on green belt land - this is in the East Sussex area - perhaps it's different in your area - also we are classed as an area of outstanding natural beauty so this may have a bearing on the decision.
I can only confirm carmalee's answer.
We too are in East Sussex.A guy along the lane has put a mobile home on his piece of land(he hopes to restore a derelict house) and the council has told him to remove it,he is in appeal at the moment.
The problem is,after (ithink) 7 years unchallenged the council must grant PP for a permanent building,so of course they will not grant PP for even a mobile home.
Sorry.
That's because the Toewn & Country Planning Act applies everywhere in England & Wales. If you are putting up a mobile home for temporary purposes it may be possible to apply for temporary permission, often for 2 years max, otherwise you just won't be allowed to do it.
It's 4 yrs, not 7yrs, after which an unauthorised structure cannot be forced to be removed.