The PCM is typically a silver metal box, looks like brushed aluminum. It's an inch or so thick, about 6 to 8 inches square. It'll have a sticker on it and the PCM code, typically three letters and a number, are printed on that sticker. Somewhere near the steering column, you'll find the harness the tuner plugs into. It likely as a rubber cover, designed to protect the contacts, and that may make it difficult to recognize. Once you've located the PCM and you get your tuner, while in the coach with tuner in hand, if you look at the end of the harness on the tuner it shouldn't be too long before you recognize the corresponding harness in your coach. It will be in the driver's side dash area, as it's used for all engine diagnostics and needs to be accessible to technicians.
To answer your last question about being worth it: I consider the 5Star tune worth every penny I paid. I absolutely love what it did for my rig. And, Mike was a pleasure to work with.