Shower drains can be either traditional P traps or "dry sealing valves".
From our Reflection owners manual . . .
"Your RV may be equipped with a dry sealing valve that prevents the escape of odors from your waste system and eliminates the need for P-traps. Should the RV drain piping system become clogged and a mechanical cleanout tool is used to open the drain pipe, it is important that the dry valve be removed before passing the cleanout tool through the piping."
How you solve this probably depends on what type of shower drain system you have.
Does anyone know which system is used on the 379FL ?