At this point, I feel pretty confident that David left on his own accord. Anything else requires a complicated and very unlikely scenario. I'm going all-in on the theory that he planned his disappearance.
It doesn't make sense that he was abducted and/or harmed in any way. No signs of a struggle, no damage to the truck, wallet and money untouched. No indication at all that anything bad happened. Hard to imagine he was hijacked in any form either - hijacking a big rig is a lot different than carjacking a car, there would be some indication of that.
A medical situation doesn't make sense either, that would require believing that instead of just pulling over and calling for help he instead chose to turn the wrong direction and go down a mostly deserted highway. Then get out, toss some of his stuff in the ditch, then something happened that he disappeared. None of that makes sense.
The only explanation that makes sense is that he planned his disappearance. And that would require only that he made it look like that's not what happened. Yes, he left his wallet and $2k behind - but we have no idea what he didn't leave behind. If he's been planning this for awhile he could have easily skimmed a few hundred or more every week from his operating cash.
Lacking anything that indicates otherwise, it appears he turned North at that intersection deliberately. And I believe that was pre-planned. He needed a place to leave his truck and disappear without being seen, turning south at that point wouldn't work.
And I believe he probably had an accomplice and a vehicle waiting. Confirmation of final plan happened when he was pulled over at Marker 126. He was only there for 16 minutes (and I believe that place closes at 11pm). He wasn't there for fuel - if he needed fuel, that stop would have lasted more than 16 minutes. He was seen scrolling through his phone, but I question whether "his phone" was the only phone he had. Quite possible he had a burner.
Why did it take him 22 minutes to just a few miles north? Perhaps he was waiting for confirmation that his ride was where they were supposed to be. Waiting to ensure that everything was in place and he hadn't been seen or followed.
It's interesting that his belongings found at the scene weren't far from the truck. I don't suspect he'd want to wander too far off to toss that stuff, not with a car waiting for him.
I have no idea why he would want to disappear or what his plans may be afterwards. I have no idea if his family or LE suspects as much. Stuff like this never makes sense, but it happens.