I did some more looking and found record of the divorce between Dave Campbell and your mother in 1984. Even though this is old information it may give you some clue as to addresses or names of witnesses, etc. Divorce records are public records and you should be able to order a copy of the file and see what it says.

CA Divorce Index
Dave Campbell divorced Phyllis A. Campbell (her maiden name not given) 27 Jan 1984, Alameda, Case File # M 083563, State File # 000332

You might also be able to find a hint of him in some association with family members back in Iowa. I tried looking at the 1940 census in Iowa for his father, Mike B. Campbell, but he wasn't on the 1940 census so I couldn't find out the name of any siblings or the town in which they lived. Maybe you know this and can look around for them there. Try using this tool to help you and looking for a Dave/David Campbell age 70-79 and changing the search location:
http://www.publicrecords360.com/california/people-search/cam...
Also....is the person associated with the above link a sibling (based on locations???).