The best campsite on Pumphouse to Radium is the Cottonwood site. It has large cottonwoods, flat ground, a pit toilet, and is walking distance to the warm spring. It is the first marked campsite river left below the spring.
I agree with Simpleman that Island is the best spot, but it is on the stretch from Radium to Rancho Del Rio, at least 2 or 3 miles below the warm spring. It is in the narrowest portion of the canyon river left in a grove of large Ponderosas.
Have a great float!
All you got to do is do it.