It's totally doable, and I think you got close. How does this mock up look?

(note that I would probably look a bit better WITHOUT the descending walls on the exterior - a flat "roof" plane conveying the absolute edge of the map.)

It's the same perspective used in all the 2D Zelda games and is especially useful for conveying vertical levels.

I've also seen a currently in development PC RPG which is using this technique in order to ensure that the walls don't obscure anything in the dungeons.

The trick is just to maintain, as you did above, 2 "wall" spaces adjacent to any "floor" space. Thus, two adjacent rooms can be seperated by, at minimum, 3 "wall" spaces. It'll look different, but it works.

In the Zelda games, and this is probably a wise guideline, the sides of the walls are treated only as surface, not extra space. A 3 space wide wall with only 1 top space is, then, only one space thick. One could try the same technique without the top piece, but I doubt it will look as convincing.