I have not stayed at any of these, but here is a starting point (I did a search for campgrounds near 97209, which is one of the zips in downtown Portland).

http://www.columbiariverrv.com/

http://roamersrestrvpark.com/ This could be a little tricky for getting back into Portland from Tualatin.

http://www.jantzenbeachrv.com/

Gee, they are not exactly cheap, though.

Again, do a search based on the 97209 zip code and check them out. Once you find something interesting let us know and we can help with routes to your convention.