What do the manufacturer specs call for?

It's easy. DON'T use RedGard on top of a traditional shower pan. Traditional style with preslope and liner, or direct bonded waterproofing membrane. One or the other, not both.

And this is why most tile pros won’t use redguard. Too many hacks have used it incorrectly and it has that reputation of showing that you’re you’re a hack if you use it. There are much better systems. And if someone who does use it doesn’t have and use a mil gauge, then you can be sure they aren’t doing it correctly.