You don’t say how old your caravan is but my guess is that it is 1998/2000. You can download a year 2000 handbook from the support section of this website (click on the logos at the top of this page). That contains details of the ESM2000 power unit and how to operate it.
If the charger switch on the unit is on it should light up red. If this is the case but the battery is still not charging then either the charger board in the power unit is faulty or there is a break in the line between the charger and the battery.
Does the battery power the caravan’s 12 volt lights etc.? If so then the 20 amp fuse through which it connects must be OK.
With the battery disconnected and the charger on do any of the 12 volt lights or other appliances work? If not this would suggest a faulty charger board.
The ESM 2000 units were supplied to Swift by Plug in Systems now Bonus Plug-In-Systems, part of the Bonus Electrical group
http://www.bonus-group.co.uk. They may be able to help you.
One difficulty could be removing the unit for repair. Swift often left very little spare cable to allow it to come out far enough to be disconnected. If this is an issue you could fit a power supply/charger such as those sold by Amperor and mount it in a front bed box near the battery.