The price difference between the diesel Sprinter and the gas Chevy or Promaster models will buy years worth of gas for most. This naturally depends on how much one drives it. Also there seems to be more (and much more expensive) issues with the new diesel systems. There is also the detail that Sprinter service centers are often far away... and rare in the middle of the country.
The Promaster platform seems to be liked (even loved) by the vast majority of those who are getting them. Seems true since we all know that the majority of posters on the net are the ones with issues to report. Most of the complaints online have related to the Roadtrek conversion rather than the van. But, that is true of all of their models. The new high-tech stuff is still pretty glitchy... seems to be a continuing learning curve with lots of user confusion and error added to the usual problems that come with new technology.
As to discounts off MSRP, that depends on the models, how long it has been on the lot, one's negotiating ability, etc. Somewhere between 15-18% seems to be the average... the more options added, the higher the discount. RT's options are hugely over-priced.