[LT7A]

Lots of ideas here. I'll make a couple of assumptions:

You'd like to clean it up to a usable condition, no immediate plans for paint or to part it out.

You will be doing work on other parts of it since it has been sitting so long.

With those in mind, I would use any standard car wash soap, mix it strong, and use a gentle car wash brush. The soap is mild but won't pull oils from the paint that you want to keep (whatever remains). The mold and other surface materials may actually have helped preserve the paint so you want to remove them gently. The brush is a stronger way to agitate the soap into the deposits than a sponge, mitt, or microfiber so it shouldn't be used on good paint where swirl marks matter but will be an ally here. The brush also speeds up the process since it increases your reach. Use a bucket for soap/water and a separate one to rinse in.

Since it will go fast with the brush, just do it multiple times. I assumed that you are doing other things to the truck so each day you wrap up the work, soap it down and rinse it off. I got it down to a 10 minute process when I had to wash my truck weekly. The paint will look a little better each time and no real elbow grease involved. After a few washings, you can use straight car wash soap on a sponge for stubborn places.

Once it is fairly clean, a coat of cleaner wax will protect it and give you a good idea of what you're working with. Then you can decide if you want to get all detail-master on it with clay bar, compounds, etc. Might yield great results or might have diminishing returns. Once the paint is clean you can figure out what's worthwhile.

If you don't expect the first round to get it all clean, but wash it a few times, you'll preserve whatever is left of the paint. There are quicker methods but they all can impact the paint (he says from experience).

Good luck. Show us some before, during, after pics. Thanks!