Francesca I found a link to a Toyota Camry Manual
regarding gas to use. Reads in part:
"Use only gasoline containing a maximum of 10% ethanol.
DO NOT use any flex-fuel or gasoline that could contain more than 10% ethanol, including from any pump labeled E15, E30, E50, E85 (which are only some examples of fuel containing more than 10% ethanol)."
A little picture in the manual shows E15 as having 15% ethanol.
The Toyota Tacoma manual may have had something the same in it as above and perhaps I just didnt look hard enough through it.
But back to your original question in regards to asking to see something from a car maker that makes the warranty void assertion - it sure looks like with the way Toyota has worded their warranty and their manuals & signage on their gas caps, that using E15 might be a real bad idea ;-)