If you mean the white plastic part, which is located at the shower knobs, that would be the vacuum break, and they are notorious for going bad. Mine did (and that shower faucet was brand new), so what I did was go to Camping World, and I bought a new shower faucet with the metal attachment instead. I haven't had any problems since. Camping World does sell that plastic part for like $2.50, but I think you'll be happier with the all-metal faucet.
1971 Sovereign International - SOLD
2004 F-350 King Ranch
AIR # 8239