Is this your owner's manual?
On p. F-31 [PDF p. 107/222], at the top, the twin-bed illustration would seem to apply to your 29' IMO ["no matter what style"], even though it is entitled Thirty-Two Foot. There is an "X" right where your photo shows the valve/drain.
If you look under the trailer, you should see the blue pipe exit straight down, and water will come out of it when you open the valve.
PS -- Edit -- Oops just found your earlier thread -- still have the following manual handy?
[click on orange arrow in quote to go there]
Originally Posted by Re-Pete
Thanks to OTRA15
I took a look at my AS
manual and low and behold in section F page 14 there was a drawing showing the Black Tank Flush Assembly
, including the Vacuum breaker. Knowledge is power, I've been curled up with the manual all morning.