As with any language, you really can't learn it from books/tapes. You can get some basics from those, though. I was fairly fluent in sign many years ago, and learned it from being involved in the deaf community. I only took one class to get some basics. I haven't used it for years, and have forgotten a lot (as with any language).
The most important thing you can learn is how to ask what the sign is for X and to be able to do the finger spelling. That way, you can learn as you need to, so you will retain it better. And then READING the fingerspelling is another challange altogether.
If you have time to participate in some activities with the deaf community while in medical school, great! That will help you a lot.
I'm not sure it's realistic to expect that you can put enough effort into it to be able to conduct an entire office vist without an interperter (while also doing well in medical school). However, your patient will appreciate you being able to get through some basics with you.
I'm going through the same thing trying to learn some Spanish now. My short term goal is to be able to get through some basic H&P stuff. However it will be a long time before I will be able to do without an interpreter.