Both latic, and reidt have good suggestions. Sadly, there is no legal camping in Boulder proper. You'll have to go a bit out of town.
Like latic said, campgrounds and forest roads off the Peak to Peak highway (72/119) up Boulder Canyon are great choices. Getting to and from the Peak to Peak to Boulder is pretty easy and a nice drive. By October many of the campgrounds will be closed, so expect to boondock. The Peak to Peak highway is up high so be prepared to bundle up at night-- fall foliage may still be in full effect.
A few other suggestions:
The Boulder County Fairgrounds in Longmont (an easy/quick 9 miles north of Boulder) offers RV camping (not in the mountains, urban). Might make a good choice based on convenience.
There are few RV Parks near Blackhawk and Central City (Google it). Its about 40-50 minutes from Boulder -- main here attraction is casinos. Probably not worth it. If your that far out, camp in the woods!
Carter Lake County Park in the Loveland area-- a little far out.
Golden has a few private RV parks.
I could list a bunch more with details, but I'm not sure exactly what your looking for given you can't camp in right in Boulder proper.
Maybe we should have a little Colorado SMB get together around that time??