You're never going to make it using an electric knife and much less a serrated one.
Besides a knife which is thin and flexible, it has to be very very sharp, that the only way you will ever achieve paper thin slices.
Ok, I'm going to get poetic here, but I look at it like if I was playing the violin. The movements have to graceful, steady and deliberate. Just like a violin player uses his bow to make music, I don't think you'll ever see them start to hit, smash, slam the bow around on the instrument, and neither should you treat your ham leg like if it was some type of log that you have to saw through (you can do that with the bone, to make a great broth).
When the knife is well sharpened, just by laying the blade flatly on the meat and pressing down gently just a little and moving the knife with long strokes left to right while you push the blade forward you should get better slices with a little practice.
If your going to purchase a knife, make sure to also buy a sharpening stone or some type of sharpening tool, the key is to make sure your knife is very sharp.
I follow up on Mongos comments, I've been cutting ham since I was a kid, and still there are plenty of times, you just can't get it right, so don't worry about it too much and just enjoy your jamon
I'm still practicing every day, it's just an excuse to eat ham, but I practice.
Even sometimes, I cut them too thick on purpose and chop them up and put them in an omolette or with peas, green beans or on a pizza.mmmmmm