I have used a product called Gel-Gloss. It is both a cleaner and a wax in one. It comes in " Reg." and "Heavy Duty" formulas. If you have a surface that has some black streaks and grim including road tar you pour a small amount onto a terry cloth rag and rub the grim off working in a 12" to 18" area at a time. Don't rub all the Gel-Gloss off, let it haze up for 15 minutes as you work down the trailer. Come back and then wipe off the dry Gel-Gloss with a clean soft rag. You will go thru a bunch of rags to do a whole trailer depending on its size. Every time you pour a dollip of Gel-Gloss out onto your rag use a clean section. After I'm all done I'll put a pile of rags in a 5 gallon pail and wash them out to reuse again. With a PU you should be done in no time. I used it on my old Coleman PU. This past weekend we were camping next to a guy who bought a used 98' Aerolite trailer. He said he waxed the trailer, but it looked yellowish and oxidized and very streaky. I brought out my jug of Gel-Gloss and did a section, which brought it back to its white color and very close to looking new.
Gel-Gloss is sold at most RV dealers. I have seen it years back at Wal Mart.....but not in the last 5 years....I'll wax my 24' trailer about 2-3 times over the season. Usually doing one side per wash. It also really helps keep your decals very bright and colorful....