LOL! Trust me ... not everybody speaks perfect English in Texas. And Texas English is exactly what I based my comment on. Perhaps you learned a lot from the folks up in your neck o' da woods, but I can't say the same for the "I seens" in these parts.
These days, in (western) PA it's primarily a rural thing, but a lot of older folks, urban and rural, use it extensively. I suspect it has a lot to do with the pockets of immigrants from various linguistic groups who were shepherded into ethnic coal and steel towns. Unlike today's *immigrants*, they insisted their children speak English, which they did without anyone around to correct their usage.
Here in Louisiana, I'm personally aware of a couple cases where a journalist would misquote a correctly, but very softly spoken, "I've seen," as, "I seen," when it served to add color or to make the interviewee look like a rube.
To bring everything full circle, I really doubt this lady saw a bigfoot, but would not dismiss her story on grammar alone...In fact, if you rely upon highly polished elocution, you could look to academia, Hollywood, or even certain presidential candidates noted for their oratory, but are among the ranks of those I'd consider the very least credible people.
As a matter of course, I'd be far more inclined to trust somebody who began by saying, "I seen...."