Again, thread-wide apologies if I've helped send us down an unproductive path. I focused on the kennel as one of the only known independent sightings of Barbara the morning of her disappearance.
If she was a frequent client, the staff there might have familiarity with her enough to know if she interacted with them in a way that they could perceive as unusual. Abnormality in this case could be possibly defined as: 1.) her reservation for the dog(s) was made at shorter notice, 2.) she declined to greet them in the same pattern, 3.) her appearance/attire differed from what is listed on the missing flyers and in the press.
The order of visitation could prove useful, as well. There seems to be a dearth of information about the time span between the kennel drop-off, the neighbor's sighting, the timestamp of the photograph that she was reported to have taken during the hike, and the 911 call.
The kennel's place in the timeline might also help us establish if they made subsequent stops prior to heading to the interstate. I'm not familiar with RVs/campers, but I have driven this stretch of road and find it reasonable to suggest the couple would have needed to pre-plan for fuel. Same for food and beverages (unless they are home-brewers or bulk-buyers, someone made a beer run). The presence or absence of Barbara on these necessary pitstops could be valuable details.
Last, her husband's focus on abduction could have been reinforced by a sense they were being followed. The VI has indicated that he feels some details have been withheld from her husband's account. This withholding could be unintentional from faulty memory or stress, or deliberate at the request of LE or another motive hereto unknown. Motives aside, any potential pitstops could have security footage that might help determine if their rig was being "cased". Any verified dots on a map can help fellow drivers (especially long-haulers) with a time of reference to review any dash cam footage.