Installing New Bathtub

I am about to install a new fiberglass tub in my bathroom (replacing an old one) on a concrete slab
I have been told by some people to use concrete underneath to support it and mortar by other people.
I'm a pretty big guy, about 350, what would be the best to use?

I appreciate any help you could give me with this
Thanks,
Shannon

Concrete has aggragate (stones) in it and that could give you grief. Deck mud (mostly sand and portland cement) or Stuctolite work well. The goal is not to actually bond the tub to it, but to provide good support so the fiberglass doesn't flex. SOme people put a sheet of plastic on top of the mortar to act as a bond breaker. The goal is good support AND perfectly level. One technique is to put a number of piles of the stuff down, then smush the tub into it to level, then let it sit, preferably overnight.

I appreciate the reply
I've also been told by some to stand in the tub after you install it. Others have said not to.
Which is better?

Standing will put spot pressure on a point and potentially bend the tub, leaving a void when you get out. The best thing is to have everything ready so you can fill it with water, then let it set. The water will provide more weight than you can, and apply it evenly over the entire surface. Deck mud, or sand mix can be purchased inexpensively and is a mix of about 3 parts sand to one part cement. The same sort of stuff you'd make a shower pan out of (although they ususally use more sand). More sand, for your purposes would make it harder to smush the tub into, so the stock sand mix is just about right. When mixed right, everything is wet and if you grab a handful and squeeze, little if any water drips out, but it sticks together. It doesn't look like much, but once it sets up, its fine. Get too much water mixed in, and it will tend to shrink and crack as it cures.

I put water in the tub, which adds plenty of weight, and lets me test the drain connection.
For mortar, I place piles that allow spreading out when the tub is dropped. You can place plastic between the tub and mortar before the tub is dropped.

doesn't putting weight into the tub "flex" it, or push it downward, (possibly putting stress on the perimeter flange if it is a deck mount)? Then, unless the mortar is bonding to the tub to hold it there, when the weight is removed the tub will revert to its original configuration, leaving a ''void" between the mortar and tub, allowing flexing EVERY TIME water or a person gets into it. I prefer to put the "grout" under the tub, and then just set it down level, then wait for it to harden, BEFORE putting water into it.