Yes, they can easily park 3 or more feet out toward the road. It's like they park this way to protect their precious car by leaving it farther from the road, but it screws over pedestrians. As far as utility trailers and campers blocking the full width of the sidewalk, they don't belong there at all, and since they are only there sometimes I can only assume that yes, they have somewhere better to park.
Edit: 3 feet is about 1 meter for folks using sensible measurement units.
The thought crosses my mind, but I wouldn't do that. What I do always try to make a point to do is walk on the sidewalk side, even if it's really tight so that hopefully they will notice someone is uncomfortably close to their precious car/trailer/camper. Hopefully they will think it's going to get scratched and leave more room next time.