I do not have access to Ancestry.com but just checking out the site, I see that there is a David S. Thomas living in Yakima, Washington in the 1930 census. It has him born about 1863, which would be just about right for the David you are searching for.
Clara, Evan S., Edith V. Frank and Phillip are living with him. It also looks like there might be a Donald b. abt. 1921 and Resse b. abt. 1927 that could also possibly be in the household.
There is a Grace Thomas living in Nez Perce, Idaho in the 1930 census. She was born abt. 1903. There is also a Harvey C. Thomas, possibly living in the same household. He was born about 1875.
Lots of luck to you William.