R&R is still one of the largest. The elitism was definitely something I saw when I wrote the RCI integration at Dealer Wizard around 2014. I've heard they've been trying to change the culture internally. I'm not sure how that's going, but it'd certainly be welcome.
Does Fortellis even accept other DMSes? I've dealt with it a bit to integrate with eLeads (now CDK CRM), but there didn't seem to be much on there apart from CDK's APIs.