Per AR 71-32 and AR 570-4, items of equipment and personnel connected to organizational requirements are collectively considered Associated Support Items of Equipment (ASIOE).  In your stated situation, you are fielding the HMMWV for the organization - and the shelter item is part of the ASIOE for the HMMWV.  The AR states "ASIOE are included in the Basis of Issue Plans (BOIP) of the item that drives the requirements - in this case the HMMWV.