Actually, assuming David Westerfield entered the van Dam's household is just as much outright speculation since there was zero evidence of his presence there. The hair, fiber, and DNA evidence only proves Danielle was in the MH, not when she was in it. That leads to two theories. The first is that a drunken 50-year-old man with no past record of sexual predatory behavior snuck into a house he'd never entered before, abducted Danielle, and then spirited her away to the motor home (presumably while still alive). The second is that Danielle had the opportunity to explore the motor home while walking to the park, to a friend's house, etc. and left trace evidence during her "adventure."
Having been a kid that was prone to exploring places I shouldn't, the second sounds more reasonable at first blush, but that's just my opinion.
But the only evidence entered at trial shows this option is not possible. Danielle was not allowed to walk across the street and such alone and there is not evidence she had any opportunity to explore the MH. The forensic evidence found in the MH is what establishes DW getting her that weekend, coupled with him being gone on a very erratic trip that resulted in no normal camping activity. JMO