Usually shower heads (both handheld and wall mount) have 1/2" NPT threads. You probably need a 1/2" NPT to garden hose adapter, which should be available at a home center, plumbing supply place, etc. At worst, you may have to combine a couple of fittings/adapters.
Wall-mount shower heads have female threads; handheld units have male threads, and the hose between them and the wall has female threads at either end. These are the standard attachments; I suppose it's possible your RV is different, but that would be somewhat unexpected.