natureboyhays: It's funny that you mention Colorado Bend SP because that was the road I thought about when I was driving down Dolan Creek road. The road is smooth dirt with some gravel and I did it with a Ford Focus so you shouldn't have any problems. If you go during hunting season like I did you're sure to get some strange looks from the occasional truck full of hunters.
I was told that reservations were required for an overnight stay so I don't think that day use would require one. It's not like they have to keep the crowds out.
Unfortunately the access to the river is somewhat limited. There's a nice big sign indicating that you have left the state natural area at the end of the trail and the rest is private property. As far as the trail itself is concerned, there's about a mile between campsite 4 - the closest to the river - and a locked gate. That's as far as you can go with your vehicle and there's a parking area there. The road leading from the headquarters is the same smooth dirt surface. After the gate, it's about one and a half miles to the river. I just walked from camp #4 instead of driving.
I would suggest an overnight as opposed to a day trip because it takes so long to get there. It's sixty-five miles from Del Rio and seventy from Sonora - those are the two closest cities. In a car like a Ford Focus it will take you at least an hour to get to the park once you hit Dolan Creek road.
Regarding Dolan Creek road; keep an eye out for it because it's not well marked (see the rusty sign in my pictures) and there's no sign telling you that Devil's River SNA is down the road.
Let me know if you have any more questions, I'll try my best to answer them.
