couch,
You can buy the maps for the Southern Tier from Adventure Cycling. When I toured that route I bought two of their maps (San Diego to Las Cruces). I looked at a lot of web sites and found that Woman Tours has an organized ride along the southern tier and they post enough information on line for you to figure out what roads they use. I followed some of their routing and then made up some of my own routing to get me to Florida. i don't think that I would use Google routing, I do better planning from a paper map. I found that by the time I made it to Las Cruces following Adv. Cycle maps I was comfortable choosing my own route from a paper map. The Google routing will most time pick major roads when there might be better options for bicycle touring.
As for low weight, that is a reasonable goal. Certainly, most tourers carry a lot more stuff than they really need. I would agree with most comments here and not carry my stuff on my back. Even if you ride a road bike for this tour you can figure out some way to carry your small amount of stuff other than on your back. I used a stock road bike on the Southern Tier and mounted two rear panniers for my stuff. A couple of small panniers don't weigh much at all.