RV coach and chassis manufacturing dates are often different. I saw a post on an RV forum by someone who claimed to have a 1981 Dodge RV with a factory 440. One of the forum members over there posted that some of the RV manufacturers like the Dodge chassis, so when they heard Dodge was going to discontinue the 440's they stocked up.

If the RV in question is a class C with the Van front sheet metal, it would be easy to tell if it's a 78 and older or a 79 and newer because the Dodge line of van's got new front sheet metal in 1979.