It's an old German fire truck
The book I recommend to people planning a van conversion, John Speed's Travel Vans, centers around the conversion of his 4x4 Vario into a highly-capable overlander. (http://www.travelvans.co.uk)I agree that it is either a T2 or a Vario (Duesseldorfer). The larger Models 814 and 815 could have a GVW of up to 7.500 kg, and empty weight only about 3.500 kg. There was even an 4x4 version available (815DA).