In a Class B, because of the windows, not great insulation, and the fact that they leak air, expect only about 10 to 15 degrees lower inside temps. You might see 20 if the van has been sitting in the shade, parked on dirt/grass, and the chassis has cooled cooled down.
There are many threads on this in the archives, and about ways to achieve optimum cooling, but 20 degrees is as low as you will get.
One big problem in Texas, AZ, and other SW states is the ground stays hot through the night. Summer here in Tucson is often in the high 80's low 90' at midnight. Hot air leaks into the van, so depending on how hot that air is, and how fast it gets in, will determine how efficient the A/C will be.