hill country/ big bend area

I would add Seminole State Park after Comstock and just before reaching the Pecos River. There you can take a tour down to an Indian rock shelter. They also have a nice campground on a wind-blown hill. If you take the road south from Marathon down to Big Bend Nat'l Park you will find a bunch of things to see there plus more campgrounds.

thanks (the texan)for all the information. will there be places to get diesel on 90?

It's all open road. Other than the Judge Roy Bean museum in Langtry, which is not all that interesting IMO, it's a whole lotta nuttin' till you get to the BB. :)
Stock up in Del Rio.

Drop down to US-90 in Uvalde and take it west to Van Horne. there are several things to see in Brackettville, the Alamo movie set and the old fort, then on to Del Rio, which is a typical border town, but is safe to visit, if you're interested. When you head out of Del Rio, you come to Langtry and the home of Judge Roy Bean, which has several restored buildings. then you cross the Pecos river and there is a very nice pull off on the west side of the road. Most of the small towns you go through have a history and they are worth exploring. The Big Bend area is desolate, but at the same time very interesting. It is high desert(2.5k) and mountains(8k+), so be careful and watch the weather. the town of Alpine has Sol Ross University and many nice old features. The McDonald observatory, east of there is worth the side trip. On down 90 is the town of Marfa and the famed Marfa lights. You can spend a week exploring the length of US-90, from San Antonio to Van Horne and still not see it all.

Roads are excellent 2 lane TX roads. As far as I am concerned, there is nothing to warn you about, as it is beautiful country. We usually stay in Del Rio and then in BB NP or the town of La Jitas. Go and enjoy a true off the beaten path adventure....