It is the anti-siphon vacuum breaker device that is leaking.
They are OPEN to atmosphere (that's how they break vacuum) so when the disc inside doesn't seal the open cap water leaks out.....especially if water pressure is high
Replace it with new one OR remove it and hard pipe it with couple elbows.
RV MFG. is required to install due to potable water and black tank system ----possible cross contamination.
But I would really like to know HOW that would happen.
Probable .....HIGHLY unlikely
add a back flow check valve on supply hose.
No backflow and no more LEAKS inside the RV