Did viewers ever see inside the den at the back of Maude's house?
If so, which episode?

While the den was frequently mentioned as a room where Walter and Arthur would retreat to escape Maude’s lectures or play cards, it was never actually shown on screen.
Like many sitcoms of the era, certain rooms—such as the den and often the upstairs bedrooms—remained off-camera "utility" rooms used to explain where characters were when they weren't in the primary living or kitchen sets.

A few notable details about the "unseen" den:

The Painting Incident: In the episode "Maude Receives A Threatening Call," Maude mentions hiring a painter specifically to paint the bedroom and the den.

Arthur’s Hideout: After a fire at his own house, Arthur moves in and often keeps Walter up all night playing games in the den.

Location: Narratively, the den was located past the back area of the house, likely near the kitchen or the back door.