It's a tough call; I think the 420hp 6.4L Hemi is going to be a beast, yet we already know the CTD is a beast.
Win/win I say...the less expensive fuel for the 6.4L Hemi will somewhat balance out the better MPG of the CTD...
Since you're buying new you're warrantied either way so I wouldn't waste any time being terrified of expensive repairs on the diesel. I think the water in fuel thing, though a possibility, is extremely unlikely to be an issue so I wouldn't factor that into your decision either.
Longevity? For me that's also a toss-up...modern gas engines can run hundreds of thousands of miles just as diesels do, so that argument doesn't hold up as it did when our fathers had that discussion. Plus, the percentage of those of us who actually keep a truck 300,000mi+ is what, maybe 1-5%?
If money isn't an issue, I'd simply drive both and make the decision based upon which one you enjoyed driving more.
As far as a bigger RV in the future:
Would the CTD be better suited to a very large 5er? Yes.
Would the 6.4L Hemi be able to handle all but the biggest 5ers? Yes.
Only you know the likelihood that you may want the biggest & heaviest of the big & heavy.