I'm guessing you haven't heard of Adventure Cycling.
I recommend you go to this page:
http://www.adventurecycling.org/routes/network.cfm
This shows a network of maps prepared especially for touring cyclists. The maps can be purchased on that web site. If you can't afford them, try putting up a request here or at
www.crazyguyonabike.com to purchase someone's used set. (visit crazyguyonabike for tour journals, lots to learn there)
The maps include specific roads, with turn by turn directions, mileages, hill profiles, locations of services such as stores, campgrounds and bike shops. These maps are very useful for a first-time tourist, because you are going to have a lot of stuff to learn as it is, and the folks who prepared the maps did a lot of research already to find out which roads are the most appropriate for cycling - most scenic, safest, best distribution of services. This is not a good-road guarantee, but it does take a lot of the guesswork out of it. If you don't like the route after using it for a while, go ahead and plan your own, but by that time you will already know a lot more about what you do and don't like to ride.
Shortness of route is a poor way to make the decision for cycling - however you route it, you are going to be out there on those roads for many many weeks - so you want to be enjoying your time, not just getting it over with. So you want nice safe roads, interesting countryside, enough services, decent weather. It's really different than in a car, when you can just turn on the A/C or the wipers, or just go an extra couple of hours.
You should make your decision about which route to take at least partially if not entirely based on the season you wish to ride. The Southern Tier which would be the most direct for your trip, is not appropriate in mid summer, you don't want to be crossing the desert southwest when it's that hot. That route would be good fall or spring. If you are riding in summer, head north and get on the TransAmerica, then hang a left when you get to Oregon and ride down the coast to SoCal. Of course, that is a LOT more miles than the direct route.
If you don't have time to do a great route between your starting and ending spots, I would suggest revising your start/end points, not trying to shorten the route. The riding will be a lot more fun if you can relax about your timeframe, have time to explore stuff that interests you along the way, hang out if you meet someone really cool, etc. (I still have an excellent boyfriend I met on an XC tour 4 years ago, for example). I know it sounds really cool to say you rode from there to here, but it's a lot more important how you feel WHILE YOU ARE DOING IT, than what you SAY about it later.
You can rent a car one-way or get on a bus to your starting location, or just ride to wherever appeals to you until you run out of time, and do the same.
As to camping on the shoulder of an interstate... it would only take one driver pulling over with car trouble or falling asleep a the wheel, and you are roadkill. There is NO WAY I would do that.
Good luck to you in your planning and riding, and keep asking questions, there's lots of accumulated knowledge here.
Peace.