Need serious help changing out the shower faucet

Hello,
I was wondering if anyone has ever changed out the faucet in the shower? When I turn it on it leaks from both knobs very bad. I took the trim off and removed the two screws and started cracking the seal so I could take a peak down at the knobs but I don't think this is the right access point. Should I try and take out the tub? If so how do I disconnect the drain? Should I access through the outside shower? This is so frustrating and I don't want to tear up the unit. I order the faucet from Adam so I have the TM replacement part. I already tried screwing the new stems in but it still leaked so I was back to square one which is replacing the whole unit. Any help or ideas would be apprecated. I really could use some of the great knowledge on this board. My wife won't go camping until I get it fixed! Thanks again for any help in this matter.

Eric Stone