Since new truck chassis buyers get a choice of engines, the engines are usually designed to be basically interchangeable. That means a different engine should still bolt up to the transmission, engine mounts are probably similar and other basics. But motorhome bodies get closely designed around a particular configuration, so it's hard to guess whether another engine will fit into a particular motorhome. You might well run into several complications in the engine swap and that each might need a $500 or $1000 solution, driving the cost well above the $11,500 for the Detroit. It's a gamble...
I would think a new (rebuilt?) Detroit would easily last as long as the remaining miles in the Safari. Despite all the innuendo, there are hundreds of thousands of Detroit 2-cycles diesels all over the world, powering all kinds of things. Newer engines are better, of course, and the Detroit Series 60 4 cycle is one of the best, but the old ones weren't all that bad.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL