I am late in seeing this post. I too am a descendant of William Prettyman though his daughter Mary. She married Henry Mason Carpenter and they are my g-grand-parents.
I am surprised and happy to discover Robert is William's father. In the transcription of the 1856 census is age is given as 26, not 36, which pretty much stumped me. 36 makes a lot more sense.
I have looked up the military service of the related families in Louisa Co. William Prettyman was a corporal in the 25th Iowa. He served with Isaac Prettyman, who mustered out as disabled. Emmeline Britt's two brothers, Alexander & Sylvester both died in the war. Alexander was killed in action and Sylvester died of illness. Henry Carpenter's uncle Charles was killed in action. Harmon Trask, another Carpenter cousin was a POW, but was released and rejoined. A few letters from the Britts & Prettyman's were handed down. Some were quite colorful.
I found a picture of William Prettyman's headstone and I have a few other details about him I'd be happy to share.
If it's no trouble, I'd love to know about Joshua Prettyman.