I think you will find that it is isn't just you that can reside in your caravan for up to 28 days - it is the caravan as well. In other words, unless you have planning consent for permanently located vans for leisure use, you are going to have move the van off the site as least every 28 days. For how long you need to do this, I'm not sure.
The fact you can prove you have another permanent address is irrelevant - if the council failed to issue you with an enforcement notice you would eventually gain rights for a permanent residence on that site - and we wouldn't want that now would we?