Simple really this is. Type in the convoy leaders unit field: myConvoy = group this;. Then, in the objective trigger (if the convoy is dead) type: (count units myConvoy == 0). Put in the 'On Activation' field: ""1" objStatus "DONE";";
If the convoy reaches the town. Give individual names to the convoys trucks and type in the condition field of another trigger: "convoyMan1 in thisList OR convoyMan2 in thisList OR convoyMan3 in thisList;" etc... Follow this up with ""1" objStatus "FAILED";"
Essentially, you'll basically have two triggers with the following 'On Activation' field and 'Condition' field:-
If Convoy Dead:-
Condition: (count units myConvoy == 0)
On Activation: this exec "endScriptWin.sqs";
If Convoy Reaches Town:-
Condition: convoyMan1 in thisList OR convoyMan2 in thisList OR convoyMan3 in thisList
On Activation: this exec "endScriptLose.sqs";
Good luck ;D!