Hi J.,
I've had the same problem with my two year old daughter. She turned two in January, so we started the potty training a little early, according to her pediatrition.
Our situation was just as you described. I'd put my daughter down for a nap, or for bedtime, and she would wake up with ALL her cloths off. We only had one "poopy" situation in her crib, but many wet ones!
My first attempt to keep her one piece sleeper on was to place a safety pin on the inside of her sleeper catching the top of the zipper.
My second attempt was to sew a "hook and eye" at the top of her sleeper, but she still could unzip it and get out of it.
Finally my husband mentions to me, "you will have to put the sleeper on her with the zipper in the back". I did have to cut out the feet in order to do this, but it worked like a charm! No more finding her unclothed, and screaming when she woke up because she was wet and cold!
Since these are flannel sleepers, and will be too warm for the summer months, I plan to take her two piece summer pajama's, sew two long ties on the bottom of her tops, and two ties on the waist of her pajama bottoms, and tie them together not a bow tie) when she takes her naps. I'm hoping it works as well as turning the one piece sleepers around.
In reference to potty training...my daughter did the same thing. She'd learned quickly to pee pee in the potty, but poppies were another story.
Since I am also a SAHM...I would try to stay close to her in the mornings, when she usually had a bowel movement. I'd start to hear her groan...and immediatley take her to the potty. When she did poop in the potty, I'd praise her, and she in turn would be so proud of herself. She's still learning, but does have the concept of it all now. I also say, "I'm so proud of you", and she repeats...I'm proud of you!
Good Luck!
Judy