Van battery keeps draining over night?

Hi. I have just brought a 1.9 kangoo van. The van started 1st time for the first 3days with a new battery. Then it died. I got a mechanic to come and check it. He said the alternator was faulty. I bought a new alternator let the van run for a hour to recharge the battery. Turned it off and run it again just to be sure it charged it was perfect for 24hours. The battery was dead again. So I bought a new battery. The van was perfect for 3days and the battery died again. I am at my witts end I read online about the r lays maybe faulty. Does anyone have and ideas or have anyone experience this before any help very much appreciated many thanks Sean!