I attached an IRS excerpt on the clean-fuel deduction below. It doesn't mention the Tahoe but it does seem to open the door to vehicles not mentioned if they have clean-fuel burning modifications, it just limits the deduction to the cost of the modification. If this E-85 engine is the kind used by some other qualifying vehicles I can see how taking the deduction would be defensible.
Deduction for Clean-Fuel Vehicle Property
The deduction for this property may be claimed regardless of whether the property is used in a trade or business.
Clean-fuel vehicle property. Clean-fuel vehicle property is either of the following kinds of property.
A motor vehicle (defined earlier) produced by an original equipment manufacturer and designed to be propelled by a clean-burning fuel. These include designated hybrid gas-electric automobiles which, at this time, only include the Ford Escape, Honda Insight, Honda Civic Hybrid, and Toyota Prius. Those designated automobiles do not qualify for the electric vehicle credit. For other than those designated automobiles, the only part of a vehicle's basis that qualifies for the deduction is the part attributable to:
A clean-fuel engine that can use a clean-burning fuel,
The property used to store or deliver the fuel to the engine, or
The property used to exhaust gases from the combustion of the fuel.
Any property installed on a motor vehicle (including installation costs) to enable it to be propelled by a clean-burning fuel if:
The property is an engine (or modification of an engine) that can use a clean-burning fuel, or
The property is used to store or deliver that fuel to the engine or to exhaust gases from the combustion of that fuel.
For vehicles that may be propelled by both a clean-burning fuel and any other fuel, your deduction is generally the additional cost of permitting the use of the clean-burning fuel.