This may help, if I understand your question properly.
I'm not sure of the of the percentage of new trailers, 5th wheels and motorhomes come to the dealers exactly matching the 'brochure for that model'....I would guess it would be pretty low...they like to have something different to show and upsell.
Now, when you get into shopping the used market you will find that manyof the units have been factory ordered (by original owners) with 'options/extras' that may not even be listed on any brochure...but added to the unit by byers request...and you can be sure the dealer will be tacking on more for 'any extra options'.
I bought my unit (privately) used from the original owner who had ordered it from the Dallas Oregon factory and he had several optional items installed...all to my advantage...as I have a copy of the factory order sheet.
So, re: if it has a black tank flush or not.... Not a big deal...they are very easy to install DIY after market or by dealer (at 4x the $).
Hope this helps
'07 WildCat 30 RLBS ~ West Coast Edition ~
Trail Air - Tri Glide Pin Box
2000 F350 DRW 7.3 Diesel Lariat 89k miles
Edge programmer, 4" SS exhaust turbo/back
Vancouver Island, B.C., Canada