Note: I moved one portion from the middle to the top, since that's
probably the only piece of interest to the OP.

The dual rear wheels provide traction (so trailer sway won't push the
truck around) and extra width for stability. Common pickup campers are
very tall and very heavy, and therefore raise the center of gravity,
resulting in reduced stability. Towing a trailer requires lots of stability.

For 2500 and 3500 series pickups, I can only judge by what I've seen on
the road. I know the weight of roof shingles or dirt by sight, having
dealt with those loads often enough. That said, my observation of modern
vs. older trucks is that the older ones look way overloaded.

You're stuck on quantity and physical dimensions of spring leaf. I don't
think that's valid anymore; based on what I've oberserved of HD pickups,
and experienced in 1500 and compact pickups, it is obvious to me that
spring technology has changed. Either the leafs are made of a different
alloy, or maybe they are shorter in length, or whatever.

My 2002 GMC 1500 has the wimpiest looking springs I've ever seen, but
they carry more load with less squat than ballsy looking springs in
other trucks. My cousin's mid-80s F250, for example, has a big leaf
bundle the way you like, but squats easier than my miniscule setup.

2000 pounds in my truck bed is barely noticable, and very easy to forget
while driving. It still looks level, and feels stable, and rides like a
car at that load -- and that's technically overloaded, with my payload
capacity rated about 1800 including my fat a.s, my ladder racks, and the
pile of junk in my backseat.

I'll try to get you a picture of the springs in my truck. One day, I
made a few runs that turned out to be about 3000 pounds (against my
better judgement*), and that DID cause a lot of squat -- it wasn't
bottomed out, but the rear was somewhat lower than the front.

*: I went to the jobsite, an hour and a half drive, and found out we
needed some loam. If the site was closer, I'd have gone back for the
dumptruck. I went to the gravel yard and asked for 1/2 yard of loam. I
got at least 3/4 yard of sopping wet mud (filled the bed, heaping over
the top, dripping everywhere). I made two more runs like that.

I suspect that, until the late 1990s, not a lot of engineering effort
was put into trucks -- they were simple workhorses until manufacturers
determined that people would pay through the nose for something more.
Now, with profit margins many times that of a car, they're worth the
extra engineering to get both ride and capacity.

I _haven't_ done that particular research, so it's easy enough for me to
believe, as I used to.