If it isn't a valve shut off someplace (I'm not sure there would be one) , then perhaps a problem with the cartridge in the shower.
One thing to double check. In our trailer, a 2006 Classic, I have to let the hot water run for quite some time before it actually becomes hot - not sure why, the pipe run shouldn't be much different in length than the run to the galley sink.
Presume you let it run long enough to be sure it isn't working?
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.