So, the rolling hotspot went dead just north of Wichita. All due to a dead battery.
It seems the Mogul, when running WiFi and EVDO, draws more power than my 12V charger can provide. I shoulda brought the 110V charger that came with the unit and plugged it into the power strip in the car. Once we got to the hotel, I borrowed one from Jason Lee, and battery levels increased when running WMWifiRouter on the AC adapter. I think the culprit is the cheap 12V adapter I got at Wal-Mart… it was $7, instead of the $40 that Sprint wanted. It was labeled as a BlackBerry charger, so my guess is there’s a current limiting circuit in there designed to keep the BB from incinerating itself, but is insufficient to power a Mogul running at full bore. I checked on JLee’s charger, and it sources 5V/1A, which is a pretty serious amount of juice.