Appears Thro Citation uses a Suburban Brand water heater.
Suspect that 'light switch' is an add on for the WH electric element......OEM switch is in outside compartment so some mfgs add another one Inside. Switches are wired in series so BOTH have to be on to energize the electric element
*****Must have water in WH Tank before using electric element otherwise you will burn it out.
As for the other switch...suspect it is the On/Off switch for the Propane
12V DC powered....did you test it after replacing that 15A fuse?
In outside compartment above the gas valve is where the set of T-stats are installed.
'Push To Reset' on cover (manual reset for High temp t-stat---if tripped no power to heat source)
Left side is for the 120VAC electric element
Right side is the 12VDC for the propane
120VAC is fed from a CB and the 12VDC is from a fused source
Voltmeter and test the circuits....
AC and DC wiring Junction Boxes are on backside of WH Tank...wires are typically 'wire nutted' and can come loose
120VAC wiring (don't forget that add'l light switch)