The Convention Center has bike parking in the parking garage onsite, as well as racks on the sidewalk outside all the main entrances. If you use a good lock (ie. a U-lock), you'll be fine.

From what I undersand, there's no tent camping allowed inside city limits, so that may rule out camping at the RV campsites. I think Oxbow is the closest tent camping site to Portland, 20 miles east of inner Southeast Portland, though you can take the bus to within six miles of it.

Have you looked at hostels? The Hawthorne Hostel would be an easy bike trip to/from the Convention Center, as would the Northwest Portland Hostel. And since it's likely to be rainy in October, probably drier too