I believe those random numbers only get generated once when the tween gets created. This code could be optimized but this is a quick solution, perhaps I can optimize it later if necessary.
Try this:
var obj:Object = {};
function doLoop():void
{
var numX:Number = Math.random()*300;
var numY:Number = Math.random()*300;
var numRotation:Number = Math.random()*360;
TweenMax.to(obj, 1, {x:numX, y:numY, rotation:numRotation, onComplete:doLoop});
}
//call function
doLoop();