oh wait! by tent trailer, do you mean a pop-up dealie? if that's the case, then i think the RV sites are fine. The Janzen Beach one would probably be the straightest shot to the Convention Center - you'd essentially take Union to MLK to Vancouver which would dump you at the Rose Center which is right next to the Convention Center. http://www.portlandonline.com/transp...39402&a=322271
But there are good routes to town from the east as well. http://www.portlandonline.com/transp...ex.cfm?c=39402 has links to all the area bike maps if you want to plan routes.