Lightweight concrete like Gypcrete is good, heavy but fluid consistency.
BUT, first, determine if the tub you are using REQUIRES or PROHIBITS underlayment. Varies greatly by type of tub.....get the manufacturers installation instructions.
Do not bed the apron. You need to have a more or less level floor. The apron supports the front, and a 2x4 ledger across the back supports that. After those items are in place, then the mortar bed issue is per instructions.