There's several options:
1) If you're looking to just pass through, the shortest total mileage is to take US95 from Needles to Vidal Junction, then take CA 62 east to Parker. Take AZ 72 east out of Parker to Vicksburg, then Vicksburg Rd. to I-10. All of these are good, high speed two lane roads.
2) If you want to check out Quartzsite do the above until you're east of Parker and take AZ 95 south to I-10 at Quartzsite. Most of the activity is over for the season, but there should still be a fair number of swap meet vendors set up on the north side of Main Street. There's also many places to camp for the night. If you're looking for a free spot you can park in the Hi Jolly 14 day free area on the east side of US95 about 3 miles north of town. It's just before you get to the large cell tower - look for the left turn lane going into the camping area. Or there are numerous (over 30) RV parks in town.
3) Or continue on I-40 to AZ 95, then go south through Lake Havasu City if you want to check out the London Bridge. AZ95 is 4 lanes with limited stoplights through LHC.
If you have a gas rig, be advised that gas is significantly cheaper in Arizona than CA - 30-50 cents cheaper when I was there last week.
