Grew up in the Gorge have been to Longview many times. If I understand you are staring there seems to me only reasonable approach (to see gorge and coast is to go down to Vancover and go east on the WA on 14 to Maryville, cross over at to The Dalles and head back west on 84 avoid old Highway not big rig friendly, to Astoria or go down and crossover south of Portland and do the Oregon coast. The Gorge is in my mind a must see and would only add maybe 200 miles and both sides are worth seeing. Hope I didn't misunderstand your question.
