The one part of the cost/benefit equasion I did not see above is the cost of the 2 454's replaced at 250K miles each as opposed to the 500K you should be able to get on a good diesel BEFORE IT'S FIRST OVERHAUL!! No kidding-I have a Mercedes Benz diesel 300Turbo with 400K on it, doing 100miles round trip to work and back every day!!
At the risk of being forward, if there is enough interest in doing this, and some unanimity in terms of engines desired, why not do a group purchase? The price should come down buying more than one engine, cruise control, etc. I have to presume that is what Airstream did when they first had the diesels put in.