I just had 2 tubs installed and uder the tub floor it seems hollow. Shouldn't sand or something be used as a base? When I step in the tub it feels hollow and unsupported. Please help.

WOW I haven't heard this for a while. Yes there should be something underneath the tubs/showers. Is there any access to get beside and under the tub now? Did they just set it and it just sitting in the stud alcove the you can still remedy this. Where I live we use and old fashioned product ( Structo Lite or nick named Brown Coat ) this come in big bags and has sand Portland cement horse hair with fillers. We mix it up thick and put it on the whole floor alcove so we can set our tubs and showers. We gently wiggle it in place and don't nail the flanges yet. Its also great for leveling the units. You can usually get this product at a masonry supply house or they can tell you where to go.
Once it sets overnight you nail the tub/shower fiberglass flanges. When this brown coat sets up its like concrete. WE are careful not to get too much around the tub drainage pipes for service down the road. When this sets up it makes the bottom of the tub feel like concrete when you stand in it. Also the tub bottom doesn't flex or move and shake drain seals My plumbing supply house will guarantee the product, but if it cracks and goes bad they usually pay for the unit only and sometimes for some labor. 28 years setting tubs in and not one cracked or failed because of a weak tub bottom. I know I would not accept what you have now it cloud be trouble down the road. I've seen wood jammed underneath the tubs and half xxx applications. Structo Lite product to get and its not expensive.
The only options now are yank them and set it in the brown coat. Or take out the end walls and pack it in that way and push it underneath the tub with w 2 by. Another is to try to use some spray foam and drill up under the floor in a couple of spots and spray. I never tried tat before and I be worried that foam expands too much and it might bow/bubble the center of the tub. Sorry for the bad news. Again I would rip it all out and reset and forever good.
I hope I answered your question well Happy Holiday!

I just had 2 tubs installed and uder the tub floor it seems hollow. Shouldn't sand or something be used as a base? When I step in the tub it feels hollow and unsupported. Please help.

This is called "bedding in" the base and Boat gave you a very good description of how it was done. If the tub floor or shower base isn't bedded in and it gives and flexes a bit when you step into it the drain seal can rupture in time and leak.
We bed out our tubs and showers in by puddling cement or mortar under the floor/base and letting it set up and cure. Good luck and thank you for rating my answer, Tom