Berthold gives us his supposed backstory, look at the main panel:
http://i33.mangareader.net/shingeki-no-kyojin/13/shingeki-no-kyojin-2529477.jpg
(chapter 13, page 16)
A silhouetted figure standing at an open window, a shadowy titan's face peering in.
The inclusion of this panel makes me think Berthold's story was something very close to the truth,
which is to say, he was at some point (possibly long, long ago) living in a village with Reiner
when titans attacked.
The human in the panel is Berthold, the storyteller.
The decision to include this panel, rather than just remaining on Berthold's face the whole time he tells his story,
makes me think the story is in its basic details authentic & not fraudulent.
Reiner chastises Berthold for telling this story. Why? If it was purely a lie they'd both agreed on,
Reiner wouldn't rebuke him. Something in this story was revealing too much.
The question then is, when & where did this happen to Berthold and Reiner?
My guess: this happened long, long ago, before humans entered the walls,
perhaps before the 'warriors' as a race came into being.
I think Berthold & Reiner were turned into titans,
then after however many years, something similar to what happened to Ymir happened to BAR.
They gained the ability to return to human form, and the ability to shift.
This same sequence of events must have happened to Eren too,
in a past he can't remember because of his 'father's' intervention.
So yes, Eren spent time as a mindless titan before something happened (he ate the right person)
and he was human/shifter.
Eren could be from another time--he could be older than the walls,
he could have been roaming around as a titan for many years.
He just can't remember any of this.