It's a backflow preventer, or anti siphon valve. We always cut them out and use an inline one (at the inlet or on the hose) and a designated hose. I've had too many of them have a sticking valve, and dump water wherever they're mounted. Unfortunately sometimes inside the wall. On our new fifth wheel, the main bath one is / was under the master bath vanity. First test and it dumped water under there with the sticking valve. A few pex mods later and it's gone. By code they have to install them, or suppose to, and they have to be above the level of the toilet. If you have one, just check it every now and then, as they can leak. It was a problem on our Mobile Scout, Tiffin, and now this one.
John and Rebecca Dickson
Emma-7 / Little John-5 / Iva-1
Full Timing Again, Rev B
2013 Ford F-350 Lariat CC LB PSD
2015 SOB TT - With OC's Awning Poles (#8)
At least in Heaven, RVs will be perfect, and I won't have to keep fixing them.