Three possible suspects.
1: The pilot is not burning hot enough, Clean pilot orifice and adjust as per the owner's manual
2: Control valve shot, There is a magnetic latching valve in the control valve body, Pushing the button you hold while lighting "Sets" this valve in the OPEN position, current from the next suspect holds (latches) it open.
3a: Bad thermocouple flame sensor. This device develops about 0.480 volts DC when the pilot is burning properly... 3B: The connection between the thermocouple and the Control valve has tarnished and is presenting high resistance.
4: You must hold the button for around 1 minute AFTER the pilot lights.
(I'd add a bit to whatever it says on the label)
This is identical (Even the control valve is basically identical) to the ones used in houses with gas water heaters, be they propane or natural gas. Control valve may be smaller in an RV is all. But operationally it is identical to a house unit.. I heated water with Gas for 30 years at home, and my parents before that.
