There are so many translations and so many usages... I'd say "chu vere" insists (since you add a word) but basically means the same as "chu" (surprise?) or "chu estas vera", "chu vi diras la veron", or even "chu vere okazis"... or many other phrase you can invent.
"truthfully?" was marked incorrect as a translation for this. Should it be correct, or is this too clumsy to work?
Not quite. I believe that ĉu ne? is just a general question for confirmation—just like constructions such as isn't it?, haven't you?, wouldn't he?—whereas ĉu vere? in addition expresses some degree of surprise or astonishment.
The difference, if at all, is rather subtle though, so in most cases both would fit just fine.