The late attempts to to throw campaigning and advertising into states late Pennsylvania and Wisconsin (which, by the now-known-to-be-correct polls, were basically unwinnable) are probably the best evidence. There's also the more squishy testimony of journalists and pundits, who reported on genuine surprise coming from the senior Romney people about the exit polls. Karl Rove's freakout on FOX last night probably also counts.
None of that proves anything. But I think the burden of proof really goes the other way: the Obama campaign clearly seems to have played the moneyball correctly here. If Romney's folks knew they were going to lose, you'd have to show that.
And I don't know how to interpret "Romney really could have won this, had things gone a little differently". If you're talking about polling could have been different had he been winning, then of course: we'd be having the same argument in reverse. Though again, the evidence seems to say that Obama would still have outperformed the popular vote margin and thus have still been running a better data-driven campaign.
If you're saying that the data suggested by the end-campaign polls could have supported a Romney win, then no: that's just wrong, as evidenced by the discussion we're having. That kind of thinking is exactly the kind of "faith-based" notion that the Romney campaign is accused of relying on. The data showed a clear Obama victory. The data was right. Period.