DIGGING GRAVES IS HARDER THAN YOU WOULD THINK
My wife's dog died last night, and she and I had to dig a grave to bury it this morning. We live in Colorado. Our yard is sandy and rocky and dry, with natural grasses, yucca plants, and cactus, probably very much like the terrain where CW buried SW "in a shallow grave."
It took my wife and I an hour-and-a-half to dig a hole that was about 3 feet long by 2.5 feet wide by 2 feet deep with a pick axe and a shovel. We would have preferred for it to be deeper, but it would have taken an hour to get another 6 inches deep, the ground was so hard. Then when we got the dog in there, it took longer than you'd think to fill the hole back up with dirt.
This leads me to believe that if CW intended to bury all three bodies, he soon found that it would be an impossible task and would have taken all day, despite the fact that he is fairly young and in better shape than me.
Putting the girls in the oil tanks may have been out of necessity, especially if he was trying to dig graves when his Ring doorbell notification alerted him that NU had arrived at the home. He may have intended to come back later, with the CW's purse, cell phone, and belongings to finish the burying.
Edited for typo and clarification.