We did a trip in thru the Mojave in Jan 2008 that included a nearly all dirt leg from Barstow to Primm NV. The tracks of the entire trip are attached.
We rode Kingston Wash which is off the beaten track for this trip. It was a narrow sandy arroyo and was a handful on the KLR.
We went over the hill south of Mequite Lake into Primm. This was very rocky.
A major highlight of this ride is Afton Canyon about 40 miles east of Barstow.
One can also do the Mohave Trail instead of the power line road we took into Baker, but we decided not to because it looked too sandy.
We had an option for a dirt road from Baker to the Kingston Wash road, but took pavement to save time.
You can get to Vegas if you go out the north end of Mesquite Valley instead of going to Primm. There are some pretty obvious dirt county roads on maps that bring you out in the southwest part of town