I usually recommend sandmix (premixed bag of sand and cement - add water and mix). Putting a mortarbed underneath the tub is wise and relatively easy and inexpensive. What's the tub bottom design? Waffleboard or skids or...? Thinset could work, but only if you had very low clearance as thinsets tend to be designed for maximum thicknesses of 1/4" for each application. If you need thicker support, then medium bed mortar and thick bed mortars are needed. Sandmix it.
If you can remove the tub to install a mortarbed and reinstall, that would be best. If not, you may need to get creative with possibly using a grout bag to inject mortar. Post some pix if you can and various people here can make good suggestions. Greg