There are not a lot of comparable vans to give you a true comparison.
When the Ford Transit is available from an RV fitter, you might draws some comparisons to similar units on the Sprinter platform.
I suspect they will be about equal in fuel economy, and in performance in the mountains due to the Ford's turbo engine. Up front cost, the gasser should be less. For ongoing maintenance, the diesel should be a bit more.
My Travato is not really comparable because it's completely different from a Sprinter. My upfront cost was about 1/2 many of the models, the mpg is the same and the performance is about the same except climbing big mountains. But it's a lot less van in many ways which you can read about in the forums. Suits me fine though - I'm a weekender and short trip taker on the east coast.
There are lots of points of contention, I'm sure, and others will chime in. I would certainly prefer a diesel if I was going to be planning on traveling so much I expected hundreds of thousands of miles. Or if I wanted a fairly heavy B with great towing capacity to boot. Really, you need to consider how you plan to use your rig to make the decision on gas or diesel.
Good luck!