I know a few people who have come up against this problem ....but, if you can find/afford a good agent to help you, some people I know have been successful in keeping their caravan for 3 years, and got planning for a house on the land too. I think this is the best you will get, temp permission. Suppose they have this rule, otherwise, whats to stop everyone with some land sticking a static van on it?
I know someone who has managed to put up a log cabin, for three years, with the intention of building a house there eventually.
I also think you could fight them with the fact that you need to live on the land to tend your livestock, but probably you would have to prove they needed overnight looking after.