Bathroom Waterproofing

In trying to find the best "Latex Primer" for my "Kerdi over drywall" installation guide I have come up with a few road blocks. I checked with Schluter and managed to get not a product specifically as a recommendation - but I was told that Mapei and Custom Building Products make latex primers that I could use with Kerdi installations over drywall compound.

I called Mapei and they do not recommend drywall in showers and said they have no product they recommend ...