Have fun. You guys are way early. Nothing much goes on until mid January. Think that place is called Pit Stop and couple miles north of I-10 on 95 and on the east side of the hwy. They are open 24/7 and sell propane, water, and a dump. That gruff old woman is actually quite nice and very efficient. Last year they charged about $25 for water fill up and dump. Varies with size of RV. I told wife it actually varies with value of RV not the size. :B
Different RVers stay in different places. Amateurs stay south of of Quartzsite on hwy 95 at mile post 99.
Everyone should go to Quartzsite once in their life. In January it will be buzzing. Scout out the main street and the parking north of the main street behind the swap meet. Those spaces will become more important as January arrives. Hard to find parking on main street.