Hi vivaro 013, this is one reason I won't use a split charge relay, you have to cover some decent mileage to recharge the battery, your split charger and your relay must restore your vans battery before it starts to give a decent charge to your auxiliary battery.
You'll get guy's saying I have had one for years and no problems? All vehicles are different and so are there alternators fitted to there vans.
If you have your own battery charger I would remove your battery every couple of days and recharge it over night, if its a leisure battery, you may be able to restore it back to a good battery, make sure the cell's are topped up with distilled water to just above the plates.
If you don't have your own charger then I would recommend you buy one from Halfords, there intelligent charger is very good.