It is not that you cannot take the the roads out of Bend to teh Willamette valley. But pulling a 40 Ft trailer will be exciting in some of the narrow roads/ turns.
Bad enough with no trailer in some spots depending which pass he decides to drive over.
But it is up to the driver. Pick wisely, Drive carefully.
The other post only mention Hwy 20 from Ontario to Bend. Which is fine - But did not mention the next step - Bend to the Valley over the Cascades.
There are few passing lanes.