I did this trip from Yorktown to San Diego in the summer of 2008. Starting late in the year is a better idea than doing it in July and August when I went, so you're good there. I too connected to the Southern Tier. I took the TransAm from Yorktown to Owensboro, KY but then went off-route thru Sebree, Marion and Grand Rivers to hook back into the Trace and the Great Rivers Route. One memorable road that I used was SR 295: beware of ticks! Anyways, I took Great Rivers into the Underground Railroad Route, at Waverly, TN. From there, I mostly went with UGRR, but did not use the UGRR between Waveryly and Parsons in Tennessee. Instead, I took 70 West into 644/691 South and traded steep, poor condition two lane roads with no shoulders for well graded, perfect condition, four lane roads with a bike lane practically the entire way. And it saved 11 miles to boot. The next day I went off-route to Pickwick Landing which had the best motel of my entire trip, a Hampton Inn. I then hooked up with the Great Rivers route at Tishomingo, MS and did almost all of the Mississippi mileage on-route on the Natchez Trace. I promise you it will be some of the best mileage of your trip. Instead of riding the Great Rivers route to its linkup point with the Southern Tier at St. Francisville, LA I was looking to 'cut the corner', meaning to go diagonally northeast to southwest as opposed to going due south, then due west. My 'intel' told me that the roads I selected to make this savings of 135 miles were fine. The idea was to link up to the Southern Tier in Deridder, LA by taking US 84 West across the Mississippi River (make the exception and use eastbound side; wider shoulder and safer), go thru Alexandria and then use SR28 and US171 to get to Deridder. Once on the Southern Tier, I stayed on route almost entirely until Mountain Home, about 20 miles west of Kerrville where I used I-10 most of the way to El Paso for several reasons: I was told to avoid the Mexican border which can be violent, I-10 is in superior condition to the ST roads in this stretch, traffic is fairly light on I-10, there is good cellphone service on the Interstate in case of emergency, there are more motels to pick from and you are actually safer, protected in the breakdown lane by a rumble strip. I chose a more direct route to San Diego by going off the ST in Glendale, AZ via 59th Street south to Buckeye Rd. into SR85 to Gila Bend. Just make sure you have plenty of water once you get on SR85. There's nothing in those 30 miles but Lewis Prison and no intersecting roads. From Gila Bend, I took I-8 into Yuma. Riding the Interstate is legal, by the way, in Texas and Arizona. It is not legal in all sections in California and just west of the AZ border on I-8 is not one of them. Unfortunately to connect back to the Southern Tier in El Centro, CA I had no choice but to ride the Evan Hewes Highway, one of the worst maintained roads out there! The piece around Plaster City and Ocatillo has to be experienced to be believed. There were tall plants growing in the road in some places! Perhaps it has been resurfaced since I was there. One can only hope. This should give you a few options. Good luck!