Need to correct some misconceptions in this thread...
(1) There are no CO2 sniffers in an RV. There are CO detectors (different than CO2) but that has nothing to do with propane.
(2) All RVs have propane (LP) detectors but those no longer have a propane shutoff valve tied to them. The propane valve has not been part of an LP detector for at least 12 years, i.e. well before this 2004 coach was built.
Rod apparently has an electronic switch for the main propane shutoff valve. This arrangement is used when the coach layout does not provide convenient access to a manual valve on the tank. It opens/closes the tank valve via a solenoid which is operated by the switch. Sounds like the solenoid may not opening, but it is also possible the regulator is clogged or otherwise defective. First I would disconnect the gas line just downstream from the regulator to see if I had gas at that point. The regulator output is very low pressure (under 0.5 psi), so very low risk there. If no LP at the regulator outlet, try again at the regulator inlet. That will be higher pressure, so quickly cycle the valve on/off to see if gas is reaching that point. If the regulator is bad, it is easily and inexpensively replaced. It is a standard LP regulator, not an RV specific part.
To answer the original question, yes the solenoid valve can be replaced. I'm hesitant to say how easily, since I can't see it and am not familiar with the specific set-up. There should be a manual shutoff valve right at the tank outlet, and this can be shut off to work on the solenoid valve, which should be immediately downstream from the manual valve. Should not need more than a couple wrenches and the usual care needed when working on soft tubing or metal pipe. Once the new valve is installed, test for gas leaks.