In most ways this is more of a question for any truck, but, since I own an F350 DRW....

Picture, this a white truck, west of Seattle (so you know it's damp). I can wash the green-slime (yes it is really green) off every bit, except that area between the cab and the box. The area is just too narrow and deep a reach for me.

Any one have a good method they use?

(there is an old saying around here, if it doesn't grow, it will mold or corrode, that is if it doesn't do both)

 

Live there and Been there Done that.

Get a helper and take a rope and tie some wash rags on it.
You want this sort of lumpy to it's not so flat and will fill the space.
Wash the truck and when you first start drop the rag/rope
in the gap with soap on it and you and the helper pull it back
and forth. Heep adding soap and water so it does not dry out.
Rince and see how it looks. If you missed spots take a long flexible
chunk of welding rod and make a loop in the end so it does not
dig into the paint and make a ball out os a rag on the end and use that
to try to get the spots.

If you don't want to do this try one of the self car washes and the high
pressure spray to get in there and clean. The green does come off.
It just takes some work.

The pressure washer idea may be the best and if you have one even easier
just watch out for the cab vents so you don't blow water back in the cab.
If you do then you get the other problem. Mold/mildew

Sean