Several. The water valve being off (if it was winterized) is a good one.. Are you on shore power or running on battery power? The ice maker ONLY works if it has 120vac, This can come from an inverter if it is set up for it (mine can be) but otherwise it's only coming from shore/generator power.
Next, if this RV is not new, a common issue is the hose that runs from the solenoid up into the freezer and the ice tray, The blasted thing on Dometic (And far as I know norcold) is often cheap plastic way less than what was on my Frigidaire and it cracks, splits and sprays water all over OUTSIDE the fridge instead of inside where it belongs.
To find if this is the case.
(note this finds a lot of errors)
First remove outside access panel. Note location of solenoid.
Now using you RIGHT HAND grab the white plastic "FRONT" of the ice maker and pull gently toward you (it should pop off)
Turn the BIG gear in the direction of the arrow, Turn till you hear the motor kick in, If the motor does not kick in problem found, no 120vac.
If the motor kicks in get ye to the outside and sit there for like 5 to 10 minutes,, You should hear the solenoid click and if you see water spraying all over.. Problem found.
This is first level only.. I can go farther but that takes tools.
Home is where I park it!