Thank you for the feedback.
Davy, yes the whole floor is concrete. It seems to have bonded. I tried to peel and it's stuck pretty good.
Brian, my thought had been that the Redgard would be the membrane. And yes, everything is mostly on the same plane. There are two slabs, laid at different times, that are uneven. I was going to use deck mud on the small one to raise it so it's the same level, then Redgard over that.
The cost of the Redgard isn't an issue. I just want to do all I can to prevent cracking for years to come. So if there's a better way, better I find out now!