Our '82 310 has a pair of drain valves in the pipes that go to the shower, behind the passenger rear wheel well.
Last year I opened them when winterizing and then when I tried to close them in the spring, they leaked for a while. I guess the rubber seals finally took hold after a few days of warm weather.
Your coach is a 345 and newer, but maybe this part is similar.
I found the drain valves in the compartment under the closet.
Airstream OCD...there is no cure!
1983 Classic 310 Motorhome.