Maximum Economic Growth for America Through the Highway Trust Fund Act (or MEGA Trust Act) - Amends the Internal Revenue Code to transfer all excise taxes imposed on alcohol fuels to the Highway Trust Fund (the "Fund").

Authorizes the transfer to the Fund from the general fund of the Treasury of the amount of money equal to the untaxed portion of gasohol containing ethanol, effective with respect to the removal or entry of any mixture after September 30, 2003.

Eliminates provision of Code stating that obligations of the Fund shall not be interest bearing, thus allowing the Fund to earn interest, effective with respect to obligations held by the Fund after September 30, 2003.

Extends various highway-related taxes, floor stock refunds, certain tax-free sales, exemption from tax for use of highway vehicles by States and local governments and for use of certain transit-type buses, deposits into and certain specified transfers from the Fund, transfers from the Fund for motorboat fuel taxes and small-engine fuel taxes, and refunds of certain specified funds from the land and water conservation fund into the general fund.

Establishes a National Surface Transportation Infrastructure Financing Commission (the "Commission"). Permits any department or agency to detail personnel to the Commission, and requires such bodies to furnish nonconfidential materials to the Commission upon request.