There are many RV parks in Casa Grande. The two I'm familiar with are the Leisure Valley RV Park on the north side of town and Buena Tierra to the south. Though Buena Tierra is the closest to the start, it's located between two interstates and the railroad tracks. Trains blow their horns at the crossing in front of the RV park several times a night.

Consider also the free camping at North Mountain Park in Casa Grande.

Leisure Valley RV Resort
Location: 6.6 miles (15 min) to the downtown start
Street address: 9985 North Pinal Avenue
Phone: 520-836-9449

Directions from Phoenix and points north:

1. Take I-10 and take Exit 185. Exit sign says "AZ-387 to Casa Grande / Coolidge - Florence."
2. Bear right on the exit ramp and go south on Pinal Avenue. Leisure Valley RV Park is 2 miles on the left. 

Directions from Tucson and points south:

1. Take I-10 and get off at Exit 185.
2. At the end of the exit ramp, turn left and go over I-10 and south on Pinal Ave.
3. Leisure Valley RV Park is 2 miles on the left.

(Directions to the start in downtown Casa Grande - 300 km, 400 km, and 600 km brevets.)
(Directions to the start at the Round Trip Bike Shop - 200 km brevet)