For maximum longevity (you'll end up with stress cracks from flexing if you don't) and quiet operation (you'll likely have squeaks as things move if you don't), you want to do this!
A sand mix (mixture of mostly sand with portland cement) works. You can buy it already mixed...just add the water. It will be like wet beach sand, and does not flow. Add enough water so it holds together when you squeeze a handfull - it should not be dripping wet. You'll need something like a hoe to mix it. Plastic on the subfloor, too. This will prevent it from having the moisture sucked out and leave it to chemically cure into the cement.
Make sure you tamp the tub down so it is level front to back and side to side after setting it onto the mortar you've thrown on the floor. You don't want to compact it prior to setting the tub down or it would be like trying to push a brick into hard earth - difficult. You can't lift it up if you push down too much, since the stuff compacts and won't rebound - you'd leave parts unsupported. It should cure overnight, then you can do whatever you want to it. It won't look like a concrete sidewalk - it will be sandy and porous. That's okay, it is quite strong in compression and you won't be walking on it.