If it was me I would just stop in Texas and do a big circle around Texas.
Course I am biased lol lol.. I would sit down and plot a general course you want to take and then you can locate 4-5 must see places along the way. Then if you have time you can add in the little places you are gonna find a long the way. For example if you want to see The River Walk and the Alamo in San Antonio, then plot a course thru the Hill Country which is more like the real Texas. Sorry but the Piney Woods, although beautiful, looks just like Michigan, not Texas lol lol