My gut feeling here is the simplest explanation is most likely - meaning that their financial issues, and side businesses have nothing to do with their disappearance.
Likely, if that truck is involved, someone was out for cash and went by afterwards to either rob them knowing they had just had a large sale (or rob the premises since they were advertising an "estate sale" which is usually to mean the person selling goods is deceased) and expected to find an empty house.
To me it all sounds like a robbery gone horribly wrong - Nathan was in the wrong place, wrong time.
If that truck is involved, the only way they were carrying two more adults and a kids is probably in the back with a tarp to cover them. If this wasn't planned, then maybe sheets, or a tarp or plastic is missing from the house and LE knows this too.
If they have footage of the truck in the neighbourhood, I wonder if they have it again on camera with something in the back once they left.