Hi Karen, Finding people after the 1930 Census is more difficult in Oregon. As you said, Alvin A. Steward died 11 Sept. 1948, Portland per death cert #9616. Source: Oregon Death Index 1903-98, Ancestry.
Maxine Luchaco died 1 June 1957 in Multnomah Co., OR. Source: same as above.
I'd request both of their obituaries (if you don't have them) from Random Acts Genealogical Kindness. Go to raogk.org/, read guidelines, choose Oregon, choose Multnomah and choose one volunteer.
After you receive the obituaries, look for a mention of where David was living. Then use whitepages.com or zabasearch.com or Google his name and birth year to find current contact info for him.
I looked for him in the Social Security Death Index but he wasn't in there. Successful Searching!