Hi
As Smudger says, it could be an alternator charging problem, or still a dud battery.
Firstly, my suggestion would be to check that the alternator drive belt is correctly tensioned and that the alternator pulley isn't slipping.
Secondly, I would remove the inline fuse to the split charge relay from it fuse holder and use the van as normal and see if that corrects the problem - but you will need to charge your leisure battery overnight. At the same time, I would the put a tester on the cable leading to the SCR from the fuse holder and see if there is power there - there shouldn't be with the engine switched off.
There are some new intelligent split charger relays that are setup in such a way that they will only allow the alternator to charge your leisure battery once the van battery is fully charged. If you have one of them there is an adjustment screw to be set to activate the relay at the correct time. If it been set incorrectly or has become incorrectly adjusted, then the battery voltage in the leisure battery could keep the relay continously activated and the leisure battery could be draining the van battery.
The last one we bought was a Smartcom off ebay from Brocott for my son's van. The relay wandered off adjustment the otherway so the leisure battery wasn't being charged at all. A quick phone call to their technical dept got us sorted no problem. I did notice that if it was adjusted too far the other way the 2 batteries could be connected all the time.
And then thirdly, if the problem still persists then the van battery needs to be checked by the supplier. The correct way of doing it is to fully charge your battery and then put it under load to check its performance. If it passes then they will need to recharge it as it will be flat.
Fourthly, if the battery is ok, then you need to get an auto electrician to test the alternator's charging output.
The van that I use has a relay that is activated by the alternator charge warning light. I don't do much mileage so have to put the charger on the leisure battery every couple of days, but I have never been caught with a flat van battery in the 5 years of using this type of relay, although I have been caught with a flat leisure battery when I have forgotten to charge it.
Spruce