Hanks says the motion going down the mountain will recharge the battery. That would require the engine to be turning over, but they are rolling in neutral so the engine is stopped. They also do not need the battery as they are going to restart the engine by popping the clutch while the truck is rolling.
When Hank and Bobby drive past the sign announcing the World's 2nd Largest Thermometer, it says it's "2 Miles Ahaead".
The back of the truck locks from the outside. So how could Dale, Bill and Boomhauer lock themselves in?