The trick is to ease things in. Don't be threatening with the camera, wait until the overnight stop, and people are settled down, and relaxing, and joking.
My best shots of people are shot with 50mm/90mm and 135mm (headshots) lenses while I was quite near to to them, and often chatting and interacting with them. Often the best documentary photos have been shot this way, using a normal lens (around 35mm/50mm/90mm/90mm for APS-C, 35mm, 6x6 and 6x7 respectively), from quite close up. You just have to look at Cartier-Bresson, Capa, Dorothea Lange and all the other legends, to see that the photographer usually does best when up close and personal.
This is one of my own images, it's shot with a RB67 (a 6x7) at 90mm http://www.flickr.com/photos/ciao_chao/5701096661/in/photostream we were at the pub chatting, and Richard was trying to figure out a use for that massive enlarger lens. It's a very nice natural shot.
Now you could be photographing as a photoshoot but still produce natual work, http://www.flickr.com/photos/ciao_chao/4952167185/in/photostream these two guys I've known all my life, so we like to have a joke and some banter between us. I was setting up my camera, and we were just chatting, and I just clicked this before they knew I was photographing them.
Your premise is right, you need to take photos of when they don't notice, but you need to do that by engaging them into conversation, to relax them and make them forget about the camera though having a joke and some banter.