FWIW, I had a shower pan leak in another RV that I could not find until I plugged the drain. Using a mirror, found a hairline crack(very slow drip) in pan bottom that was not readily visible from topside until load was applied(person standing in pan). It then opened up with a vengeance. Fixed it with Eternabond tape.
I'm not familiar with the Ace, but hopefully there is a panel or a way to a access the drain and bottom of pan without the using the Sawzall.