Riverside is within easy walking or biking distance to everything. Sites are nice but on the small (narrow) side. They don't "officially" reserve specific spots but you can definitely ask. Take a look at the resort map to see which sites would likely work for you. Google maps on satellite view zoom in pretty well on the RV park. Reserve now to ensure you get one of the spots in the "lower" area. We like the sites in C8-C12 and D1-D5 the best as they are probably the longest sites and have a nice green space behind each site. Riverside is good example of a typical RV park.
Whistler RV park is approx. 10 miles south of Whistler so you would definitely need to "drive" into town. The sites are slightly larger than Riverside although mostly dirt. Spectacular views of the valley though as you are kind of up the side of a mountain looking across the valley. Whistler RV Park is more of a rustic RV Park (kind of like a State Park).