You almost have it right.While dealer added items will reduce payload in general that reduction will be negligible. What you really need to consider is that RV towing is like no other recreational towing activity. A boat,jet ski,or even a utility trailer will not impact the towing parameters in the way an RV trailer does. WE have not mentioned frontal area which is a parameter seldom mention but has a dramatic impact. In the end RV trailers are designed to be tongue heavy to ensure stability.
The heavy tongue suck up payload in a hurry like no other recreational trailer.
Unless one specifically seeks out and orders their 1/2 ton truck with specific increased payload packages if /when available. Most 1/2 ton trucks will be limited when towing an RV trailer.
Their really is not a huge discrepancy in the ratings, but there is discrepancy in understanding the ratings. There is also debate on the value of the ratings.
In the end either you believe the engineers and designers or you don't.
Your Sequoia is a nice truck but it is far from a great RV tow vehicle due to its lack of payload and soft suspension. I prefer to be totally relaxed while towing. I also don't want to be concerned about having too much cargo. I don't want to fear that having full tanks can affect my towing performance. I don't want to fear that everything must be just right in order to have a relaxed trip.
We all have are comfort level. I don't believe the sky will fall in one exceeds a rating. However I don't believe in ignoring the ratings either. A combo that is within ALL of its ratings will perform noticeably better than a combo that is not within its ratings.
Tow your trailer with your Sequoia, than tow it with a 3/4 ton vehicle with ample payload. The experience will be enlightening.
At some point you must decide what your priorities in a vehicle are and how dedicated you are to RV'ing. Only you can make that decision.
While research and discussion will help you make a determination, ultimately you hold your own cards.
Thanks for this reasoned response.