Need more facts here....Being a Class C, it is likely a GM, Ford or Dodge....It could also be a Toyota, which made small Class Cs. I dont know the configuration of those engines. I do know that a GM Quad 4 engine would mix the coolant/oil when the water pump fails. The Toyota may too, who knows?
Anyways, from what was said in the original post, the mechanic either changed the oil because he had to or changed the oil as an extra dollar maker....Either way, he changed the oil and caused it to leak by the sounds of it. I would be having a lawyer call them up
That is, if the story is as told