The Fairgrounds is easy to find - in fact we suggest you don't use your GPS because it likes to send you down Campus Way which is a LIMITED VEHICULAR ACCESS "road" owned by Oregon State University.
From I5 there is ONLY ONE Corvallis Exit. When you take the exit from I5 you need to head west toward Corvallis (NOT EAST toward Lebanon). Corvallis is approximately 11 miles from I5; the name of the road you are on is Highway 34/20. When you cross over the bridge and enter into Corvallis the road name changes to Harrison Blvd. Stay on Harrison Blvd all the way through town it will intersect with 53rd Street at a stoplight. Use the left-hand turn lane to turn onto 53rd Street. The Fairgrounds will be on your left in less than a quarter mile.
If you want to avoid driving through town, you can take the HWY 34/20 ByPass before entering Corvallis. There is a stoplight with dual turn lanes for the By Pass. Once you are on the ByPass, you will be driving parallel to Harrison Blvd (read directions above) so that when you get to the stoplight intersection with 53rd Street, you will need to turn right on to 53rd. From there the Fairgrounds is a little less than a mile and will be on your left. This route has a traffic circle and dips under a train trestle (which can scare you if your rig is tall - but you have to be over 13' 6" tall to not be able to pass under - the road sign says 13' 4" and the trestle sign says 13' 6").