today i lent the bongo to my brother and his family to transport them and their kit to the airport , i then get a phone call after they have done 4 miles

, the bongo had overheated , so i went out to them , all around the leisure battery and underneath the van was covered in thick oily sludge , with caution i opened the expansion tank that was full of the same liquid , had the AA tow the van to my house where it sits

.
what do you think has happened ?
any advice would be appreciated