The alternative class feature decisive strike (Player's Handbook II 51), in part, says, "As a full-round action, make one attack with an unarmed strike or a special monk weapon, using your highest base attack bonus but taking a –2 penalty on this attack roll." In other words, an attacker takes a full-round action to make a decisive strike, and it is a unique full-round action: A decisive strike is a new option the character has because of the alternative class feature.
A martial maneuver's Initiation Action entry lists the action type necessary for the maneuver's initiation (e.g. full-round, standard, swift). Because a creature typically can't—for lack of a better term—layer simultaneously two incompatible actions (e.g. a full-round action to make a decisive strike on top of a standard action to initiate the maneuver mountain hammer), a martial maneuver that's a strike can't typically be combined with the alternative class feature decisive strike. (A martial maneuver that's a boost usually can, however.)
On the other hand, the feat Stunning Fist (Player's Handbook 101) can be used "before you make your attack roll" and doing so has no associated action type. Declaring that it's being used is, essentially, not an action. This makes the feat Stunning Fist compatible with the alternative class feature decisive strike and likewise compatible with any appropriate Tome of Battle maneuver that require an attack roll.