Hi Samantha! Well, I think I found a few things for you. This is a tricky one, because Mosley is messed up so much and William is so common!
First, a couple of warnings. All I am about to say needs firming up. But after a couple of hours of working on this, I finally got a breakthrough. You will need to get more documentation to be sure. Second warning: there's a family listed on World Family Tree and the Ancestry trees that is, I believe WRONG. They have the your William Mosley associates with the wrong parents (James and Americus). So be careful if you look at that. And finally, what I'm sure you found out is that there are several William Mosleys out there. Again, be careful.
First thing I found is William's WWI Draft registration. He's in Honey Grove, Fannin Co. on Sept. 12, 1918. Addie is listed as his next of kin. He has blue eyes. His birthdate is listed as January 1873. And most importantly, he does not sign, but only makes his mark. This is important because there's a William A. Mosley floating around that is a bookkeeper in Arkansas in 1900. Since I think a bookkeeper would have to read and write, we can eliminate him as yours.
I finally found who I believe is William in the 1900 census. He's living with his brother, Jim A., in Comanche Co. The birthdates/ages are really messed up (math was not the enumerator's strong point). William is listed as born Apr. 1876 and 38 years old (see what I mean about the math?)Jim and his wife, Etta, have been married 12 years (that's wrong too, more later), and have Bennie (10), Maggie (8), Henry(6), Mattie (4), Meda (2), and James C. (1). Jim is a farmer, William is a day laborer and neither can read or write. I think that is your William.
I prowled around the 1880 census, but at first there were too many to narrow down. I found an interesting William and Jim in Burleson Co., sons of Asa and Elizabeth; and another interesting set in Navarro Co., sons of Sebron (a widower).
Long story short, I found the WWI Draft registration in Comanche Co. for James Calvin Mosley/Moseley, whose next of was his father, Jim Moseley/Mosley, and found the Texas death certificates for Henry, who was born in Corsicana (which is in Navarro Co.),and ta!da! Jim A. Mosley. Jim's father is listed as Seb Mosley. Looks like the Corsicana James and William is right.
I also found Jim and Henry's tombstone: link is at the bottom. They are buried in the Hazeldell Cemetery in Comanche Co. Jim is buried with his wife, Fietta, and it says they were married in 1896 (that would make the 12 years married wrong).
Hope this helps. And like I said, it might need a little tightening up, but I think it's right.
Oh! One other little interesting tidbit. At the Family Search Texas Death certificate database, when I ran a search of Mosley's and Comanche Co., a Charles Franklin Mosley poped up. He was born in Comanche Co. in December 1896, the son of William Mosley and Mary Kendrick. He died in Stephens Co. Texas in 1966. I think this might be William's first marriage.