I was surprised to learn that mobile homes could have basements. They do not have them where I live but neither do houses. In my area, a mobile home can be different heights above the ground depending on how it is installed to get it level. I guess a room of sorts could be built under a higher one and hidden by the bottom trim stuff. I don't think the murders were a robbery gone bad. Logically, if someone messed up robbing the first mobile home, he/she would not go to the other ones. I believe this was a planned extermination of an entire branch of the Rhoden family. I think GR was killed simply because he was there I still think KR's murder was one of opportunity by someone who learned early in the morning of the other murders and decided to rob KR and it would look like it was part of the mass killing.
I have wondered about KR being a separate crime early on.