I would pull the the front off thermostat and disconnect one of the wires. He might have to remove the thermostat entirely to get to the screws holding the wires.
If it keeps running there is probably a solenoid that is stuck in the heater. A smack on the top of the heater with the palm of his hand might dislodge it. If not he'll probably need a repair tech.
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2001 Suburban 4x4. 6.0L, 4.10 3/4 ton **** 2005 Jayco Jay Flight 27BH **** 1986 Coleman Columbia Popup