The truth is that living in a tent isn't going to be comfortable at all.
Not eating or drinking, you're going to die in a couple of days. The human body can typically manage 3 days without water. When you factor in environmental exposure and physical activity, that window gets smaller and smaller. Be sure to let someone know where you're at so they can recover your body.
The ratings on sleeping bags are an estimate based on lab testing. Like anything else, they're not going to work as well in the real world. On the low end of temperature, you also will need a heat source and your tent needs to be suited to low temperatures. This is problematic for you, since you're not likely to have the energy to gather firewood regularly since you're not eating or drinking. But that's okay, dehydration will probably kill you before you freeze to death.
On the high end of the temperature spectrum, or bodies have a number of ways to deal with high temperatures. The most useful of these relies upon being properly hydrated, though. You could deal with the heat some by getting in the water, but this is temporary and cannot be sustained all day. So if you're really dealing with 100 degree temperatures and drinking no water, you will likely die on the first day.
There are very few places where you would encounter both of those temperature extremes throughout the year, and in most of North America you'll rarely be at the extremes. If you plan do to this, I recommend the high temperature option, since it will be over quicker. Our gene pool is rooting for you, get out there!