Welcome to the forum weluv2camp.
It sounds like most everything is working ok since it runs on gas if you manualy light it. Have you checked the electrode that provides the ignition spark? The electrode should be 3/16 inches above the burner tube, the ceramic insulator should be clean, and the wire from the electrode to the spark transformer should not be pinched on any metal things. It seems the thermocouple is working ok since the gas stays on and runs. If the thermocouple did not sense the heat from the flame it would release the gas solenoid and the flame would go out in about 30 seconds. Note, in the older models like mine and earlier there is a seperate thermocouple and spark electrode, newer units have the flame sensor as part of the spark electrode.
Normally when you start it in gas mode you will hear the "thunk" from the gas solenoid opening and then you will hear a series of "snap" sounds as the igniter sparks from the electrode to the burner tube.
Gene & DW Ginny
2002 Sunline T-2363
2008 Toyota 4-runner 4wd 4.7L V-8
Reese Dual Cam straightline - P3 Brake controller