After reading my own post a couple of times, here's my theory (assuming that DM would do this, despite having enough money to buy the truck):
DM and MS fantasize about stealing a truck and chopping it. They know the 5.9L Cummins Ram is rare and worth a lot of money, so they go about finding a worthy candidate. They buy a burner phone, make a few calls, and finally settle on TB's after the failed test drive with the first guy. The plan is to take the test drive (hopefully without TB), and if he insists on coming, take his phone from him and kick him out in a remote area where it will take him a while to reach a phone to call the police. They would then take the truck to the waiting trailer, and they're gone. Unfortunately, TB fought back, and was killed in the struggle, likely by MS (as I don't think DM has it in him, and MS is more of a "




" type). They panic, and attempt to get rid of all the evidence.
I think this theory works up until here... and then it breaks down. The evidence was left in such a way that if DM was identified SOMEHOW, he was toast. All of those properties were registered in his name (and I know this because I have electronic access to the land registry, and I actually identified the Roseville Road farm as his 2 days before it was reported). They scattered evidence all over his own property, and left it out in the open. If DM was an extremely stupid, ignorant person, I could almost believe this part.. but there is no indication he is that dumb. I just can't figure this part out.