A lock is good for only one thing, and that is to keep an honest man honest. If somebody wants to steal your ride, and you make it possible to do undetected, your bike will be gone sooner or later. A hand-held electric drill can open the Kryptonite U-lock in two minutes. Cables and chains are cut easily. When you go inside to eat, seat yourself where you can keep an eye on your gear through a window. I have done this many many times. If you have the kind of bike people are looking for, and bike thieves know brands and prices, keep it with you or lock it inside if possible, and avoid leaving it locked on sidewalks especially in cities. Sleeping in campgrounds presents a problem. See if they will let you lock it inside somewhere. You can lock your bikes together, if more than one, and place them directly in front of the tent where they are visible through the netting on the entrance.
Many people might think bikes are stolen by stealth, and many are in fact stolen that way, but there is also the grab-and-run method by which the thief knows where you are inside, and even if you may be seated and watching, he will guage how far down the road he can get before you are outside and trying to stop him. Sure, you might see him take your bike, but he is 100 feet away and accelerating by the time you are off your duff and over to the curb. Call immediately and the police might arrive in forty-five minutes. Meantime, Scudgemo has himself a $1500.00 bike in the trunk of his car and he is careening through the outskirts of town. The lesson here is this. Even if you can see your bike from your place in the restaurant, keep it locked and keep your most valuable possessions with you.
As for myself, I have not had problems with thieves on long bike tours. You should be safe, but crimes and thievery do happen. Use caution and you will reduce the chances of your becoming a victim of theft.