Think I've found part of an answer.
Seems that you can't use the DVI and DP ports at the same time and what you would need (assuming it's compatible is the e port+
https://www.dell.com/community/Latitude/E5570-Docking-station-PRO3X/td-p/7530927
The new monitors don't even have DVI capability (for some reason they do have VGA), but I'm only trying to use the DP port from the dock with the splitter and/or daisy-chaining. That's where most discussions talk about the MST capability.
It's funny, I have that very page bookmarked, as it seemed to have some of the better answers. I was thinking the e-port + or +II might be the answer, but was hoping for something clearer (esp release dates so I can make sure I find something newer than what I have). When I search ebay for e-port +II, I see a lot of PR02X's, which would seem to be a step backwards from a PR03X.
Dell makes everything very confusing.
ETA: I think I'm getting a handle on this. I think the PR02X is the wider T shaped dock (with 2 sets of DVI & DP ports) where the PR03X is the narrower model (with 1 set of DVI & DP ports). Both are compatible with my Latitude E6540.
E-port + and +II seems to be available on both of those models, the most noticeable difference being the blue USB 3.0 ports on the back.
I guess the question is, do I need the double video ports to do what I need, or is the later e-port +II enough? The PR02X isn't so big as to be an issue.
Finally a decent comparison, though it doesn't mention the "plus", though I'd hope that is above the II

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