Poor distance vision from perfectly healthy eyes?

I'm 25 and have slowly been losing my distance vision for about 10 years. I've been to 13 ophthalmologists and still have no diagnosis, and every test imaginable has shown that both of my eyes are perfect in every aspect. Machines read no prescription and corrective lenses have no effect. Could it be in my brain? Only thing I haven't seen is a neuro-ophthalmologist, but they are extremely hard to find and charge a fortune. What should I do?