Craig, how have you tried to re-seal the door?
If you have put mastic around the door frame which on my van has a very small protrusion of only a few millimetres it would needed to have been spotlessly clean as the new mastic will not form a proper seal on old existing mastic. The only real solution is to remove the frame completey and spend hours cleaning it up, and new mastic to form a new seal, not a job for the faint hearted but if you have good DIY skills it can be done.
This is only my opinion and not a professional one.
What mastic did you use? Is it a dedicated caravan one? (Sikaflex / Soudal)
I did read some where that an SMP tape should be used for door frame's but check this info out before starting work or may be one of our more quailified members could confirm that one.
Have you checked the rubber seal on the back of the door?
How old is your van?
Good luck.