Motorhomes were built around a truck chassis....Dodge / GM / ??? ....With that being said.... What ever engines were available in the that chassis can be had in what was being built around it. So, IF a 413 was availble until 1977 in the larger Dodge trucks than surely we must believe that A MH could have been built using a 413. I'm not saying they were or not....Just pointing out that MH's are built around existing / available chassis' and their engines - Not the other way around. I "Never say Never" afterall..How many times has there been a car or option discovered that everyone said "Never" existed.