Lots of good information here, and perhaps I can add a bit more for what might have been inferred but not said.
Owner pride, sometimes with blinders on, can massively influence what is said about any given vehicle in regards to many things, including reliability.
My personal experience in seeing this goes way, way back as far as the 60s and 70s with VW bug owners who claimed they were the best and most reliable that could be found compared the American "crap". Of course, while the rest of us were driving around and having fun, they were adjusting their valves or freezing in the cold, if not a more major repair. In the 90s the bug folks got replaced by the Honda snobs who went through the same routine, although their vehicles were much better than the bugs and often better than the American made stuff at the time.
That said, it still goes on today but probably at a lower level than in the past because all of the vehicles are made better and last longer.
The most reliable and cost effective car I have ever owned was a 92 Escort wagon, yep, really an Escort. It ran for 18.5 years and 210K miles without major repairs. Sure it broke down rarely, but never left me stranded. Brakes, front end parts, radiator, O2 sensor, struts, rear springs, alternator were about all that got replaced in that long time of daily commutes in the horrible Minnesota weather. If I compared the cost/mile of it to friends Honda and Toyotoa of the same era it was better than all of them, which surprised them. It finally rusted out so bad it started to sag in the middle and broke the rear brake lines so it drove the junk yard on it's original, still not burning oil, engine, and getting tired transmission.
We now have a 2009 Honda CRV that has about 65K miles on it only and has only needed brakes and a catalytic converter shield. We have the 2007 Roadtrek with 60K miles on a Chevy chassis that has had a rear axle replacement and front wheel bearings, but not anything else except useful mods. I drive a 96 Buick Roadmaster wagon with 145K on it that I just finished doing a full resto on. It had given me 10 years without any major issues before the total restoration. I know nothing about the previous 10 years. I hope it is good until I don't drive any more as it is essentially new in nearly all ways.
I base my reliability guesses based on how often and how badly vehicles have problems. If it leaves me stranded, that is a really big deal. If a trouble code comes up and it makes it home, not so bad. Engine failure before 200K is not good these days. Transmission before 150K same things. Nickle and dime repairs (especially if you are paying a shop to do them) very irritating. Expensive parts (we have a Honda so I know all about that) is bad as it just sours you on the brand when the parts are twice the cost of the same parts for other vehicles.
I also read the local Craigslist car adds regularly, and find that it pretty good at showing up which vehicles have what problems. The cars on there are less expensive and higher miles so they are more representative of real world life. It showed the Subaru head gasket issues years ago. It has since showed the 2010ish Honda engine oil burning issue. It showed the Kia engine failure issue that later had the huge recall long before it was common knowledge. Currently I am seeing lots of 75K-150K engine failures on numerous vehicles and models so watching closer. I am starting to think that long oil changes and the changes away from antiscuff additves in the engine oils may be an issue.
All of this considered, I have not seen anything to indicate the Promasters have any major reliability issues, which kind of surprised me because the drivetrain is hauling a lot more weight than it does in a minivan. Kudos to Dodge on that one. The old Chevies and Fords run a long time and are abused badly by tradesmen. Not uncommon to see 300K units with only a transmission repair listed. Not a lot of Sprinters show up, but a relatively surprising amount have had engine replacements and even more transmission replacements by 150K. Transits are just starting to show up there and seem to be doing very well at this point. The messenger and delivery ones can easily have 200K of tough miles on them now.
Personally, I don't think I would consider either a Transit or Promaster proven to more reliable. I do think the way they drive could be different though as front drivers and rear drivers just behave differently. Personal choice of what you like. The Transit with the Ecoboost is going to sway the decision for a lot of people I think. It is much, much more powerful and having turbo boost will make it not lose much at high altitudes which can be a big deal in RVs, It think. The Chevies are really on the used market so not much there to think about if you are looking at new.
When folks spend a huge amount of money on an RV they have a lot of ego tied up in them, so be sure to consider that when reading opinions and reviews. "Professional" reviews are usually bought and paid for ads, so mostly ignore them. If you are risk aversive, as I am, these kinds of decisions can get very hard to do, and if you are not serious car nut or techie it can be really easy to get influenced by inputs that might not be truly the best for you.
Good luck on your quest, it can be a scary and confusing trip. You are lucky, I think, though as the newer vans are much better than they were 15 years ago, regardless of brand. Concentrate more on the RV build quality and usability as that is what you much more likely to have problems with.
