Replace the bushings first, they commonly wear out, the springs don't.
You need a special tool to replace the trailing arm bushings, I built
one from plans I found online but then I had access to machine shop so
it was easier. You can build one with more basic tools if you need to
though or have a shop replace the bushings.

You're not going to find out for sure unless you remove the bolt through
that bushing and inspect it, or crawl under a non sagging 240 and
measure the length of the spring, and compare it to yours.  Sagging
springs are quite common.

Even if the bushes are worn, they don't make the rear sag. We're talking about
1/2 inch if all the rubber in the bishing is gone. More likely the springs are
worn out - fatigued.
Having dragged caravans for the past 20 years, I have had to replace springs a
few times. There are springs that are harder, however, that may have an impact
on comfort when driving without load on the rear. The alternative is to mount
pneumatic shockabsorbers that can be adjusted automatically or manually.

/ Johan