I've made roughly the same thing before without using wall priorities.
Just make the the gable end wall out of two walls: first a lower one (let's say 8' tall, or whatever your soffit height is at the flat soffit) and then an upper wall to fill in the gable (probably you'll put it on the next story up). Then make the surrounding walls on the main floor also 8' tall.
Let's say your top plate is at 12' height. At the upper story, stack a 4' tall wall above the 8 footers to make up the difference.
Make any sense? I'll attach a screenshot from the 3D window showing the lines between walls (these clean up when you render).
Of course, you might prefer for the lowest walls to simply be 12' tall or whatever, in which case you'd need Jared's methods.
cheers,
JB
AC 21 (8002) & 22 USA
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