[sourdough]

Every vehicle weight will vary by any number of things whether installed by the manufacturer or the buyer, right down to a cup holder. It would be impossible, and impractical, for any manufacturer to try to build an algorithm that adjusted every change to a vehicle a prospective buyer might make then make that instantly available via some interactive system.

GM does a terrible job IMO and leads folks to make errors; Ford isn't much better. Unfortunately that leaves any prospective buyer in a lurch and requires them to be able to "read between the lines" and understand how it all works. The bad thing is that folks just trying to get into towing don't have a clue understandably and that is 100% on the truck builders.

RVs are the same way. Built in so many ways, there are a zillion ways a trailer can be loaded....and loaded with who knows what that varies from owner to owner.

How to even hope to navigate that? Hope, educate yourself and study, study, study. Always use the highest weight numbers available (gvwr) because one day you will get there. Always give yourself some "reserves" (I like 10%). Scale your rig and the list continues.

As far as how to know the payload of a "not yet built" truck, you don't. Use all the available data to try to get as close as you can and always err on the high side when calculating weights. Never use "max towing weight" for any vehicle just like "unloaded/dry" weights; meaningless.

The "goal" is to be safe, not "get by", "cut corners", "make excuses" and then hope.