Bulletins such as WRC-107, -297, and -497 provide a method to determine the stresses in the shell due to the mechanical loads imposed on the attachment. Some of the bulletins address pressure stresses as well, and consider stresses in the nozzle neck also. But the main point is that the bulletins provide a method to determine stresses based on the specified loads, they do not provide a method to determine what the loads should be.
Generally, the loads acting on the nozzle may be specified by a design code or vessel owner's specification. Or they may be unique for the application; in other words, a 12" nozzle on one tank or vessel will have different loads applied to it than a 12" nozzle on a different tank.
ASME Boiler and Pressure Vessel Code, at least in Section VIII, doesn't specify any particular loads to be applied to nozzles. The applications for the end processes are just too varied for there to be one "magic" one-size-fits-all set of nozzle loads.
Now, you could "back in" to a set of allowable loads (forces and moments) for a given nozzle based on limiting the stresses in the nozzle and shell to the allowable stresses. But since there are 3 possible forces and 3 possible moments there will be an infinite number of possible combinations of loads that will all stress the material to the allowable. You can make some assumptions on how the forces/moments might combine and so be able to come up with a practical combination. But if you are a vessel fabricator I don't recommend "volunteering" to find this magic combination unless you are getting pad for each iteration.
Practically speaking, you should consult with whoever is responsible for attaching whatever is going to attach to those nozzles and ask them what loads they expect to have, then use these in the design/analysis of your nozzles.