I'd go with a 25 degree bag and liner or even a 15 degree bag if it isn't too much more weight. It will be fine on cold nights and on warm nights, just sleep with the liner and the use the bag for an extra layer of comfort underneath you. You need to plan for the coldest weather possible though, go to weather.com and check out the averages & record lows for the areas you will be passing thru.
Once you hit Eastern Texas, the extremes between day temps and night temps are much less than in the mountains and desert.
Lordsburg NM for example on April 18
Avg High 80 Avg Low 38
record high 94 record low 21
El Paso TX on May 1
Avg High 82 Avg 56
record high 96 record low 38
Also remember that for each 1,000 feet of elevation, it is around 5 degrees colder. So if Lordsburg sits at 5,000 feet and you are camping at 8,000 feet... it's going to be 15 degrees colder on average and more on a clear night.