Easy as pie for him to do that. All he had to do was pull his car into the garage, if it wasn't already there, and place the body in it. Since garage is attached, no witnesses. (How incompetent is the SDPD that they didn't have forensics check out his suv before he quickly dumped it?)
Not the same for DW. Assuming he was able to get the body out of the house without being seen, he would then have to carry it out to his vehicle in broad view of anyone happening by.
We don't know that they didn't check his van. BUT, they may have chosen to ignore any thing they found, BECAUSE THEY HAD THEIR MAN. Any thing they found in the van WOULDN'T SUPPORT THEIR CASE AGAINST DW.
Now, I really don't think the SDPD are all crooks. But, they could have been led by the nose, by a friend (Diane Halfman). They also could have been innocently ignoring things like that, because at the time, they were helping PARENTS look for a LOST or KIDNAPPED child. No reason, AT FIRST, to suspect the parents.