I don't think the issue is the fender, so long as it is a good tubular design metal fender aluminum or steel. I think the issue with that much weight will come down to the way they're installed. I have steel tubular fenders on my wrangler and in the rear, they mount via rivnuts inserted into the preexisting holes in the tub when you remove the factory fenders. I only had to add 2 extra holes to drill out and add rivnuts as well. So the stress will be on the weaker point which is the rivnuts and the tub itself. I personally would buy the dedicated steps that mount to the door hinges instead of standing on your fenders. I'm sure they can hold the weight, but I can see the tub flexing and you loosening those riv nuts over time and in turn your fender sagging a bit.
Have you considered something like this instead?