The P0721 (most recent error code) is interesting. Found several references saying that it's related to shaft speed sensor signal. Starting with the sensor module itself, other downstream problems could be wiring issues, etc. before it gets to the TCM. Sounds like this kind of error could be a "start veto" that would keep the starter from engaging. Another key symptom noted on several sites - could be loss of speedometer!!!, which has been our case since we've been running (we've been using my phone with a GPS speedometer app, duct taped to the dashboard, to give a large digital speed display). Speedometer has never worked since we picked up the truck. Seems like several symptoms, and all of your deductions, are pointing us in this direction.
We're safe for the night, shutdown in a parking lot a block from a hotel. Plan is to get back online and research more tonight, and be out here at first light to trace from that sensor downstream. If we need some professional help for hire, we're in the middle of Salt Lake City with more resources, and we have a better idea of what to focus on.
Thanks so much for everyone's help tonight, we really appreciate your prompt response and good ideas!! 700 miles to go...we can do this! Securing till daylight...
Sent from my 0PJA2 using Tapatalk