Older, white trailer is yellowing

You're right beem! Sorry, but I'm not sure the material. Here's a pic:

http://imgur.com/PFAl6MI

You can see the white of our truck for comparison.

Kind of critical to know what kind of siding.

Filon?
Fiberglass gelcoat?
Painted aluminum?
Other?

No on the carwash as well as pressure washer in general. What about a fine polishing compound? Check in an inconspicuous place. Then a good waxing. Suggest using 303 on your decals to freshen and protect. It'll likely never look new, but can look like a well maintained older trailer which is a good thing.

Car wash will do more damage then good.

Older RV's/Boats are sometimes renewed with acrylic polishing products - worth a look. Suggest you use the forum search feature for the term Zep - it's an inexpensive acrylic floor polish that many RVers use - not sure it will work in your case but it's not expensive and wont' take much time/money to do a small test.

TRy this: Go to one of the big box stores or a paint store and buy a gallon of Jasco prepaint conditioner. You will also need a garden sprayer or large spray bottle. Mix the solution to be used 50-50. Spray down your trailer with this mix and let it sit on the surface for 5 minutes. Scrub in and them rinse. You can then wash and wax the trailer with any good stuff like Marine products.

What you want to remove is the oxidization in the surface of the fiberglass or gel coat. Jasco is a good product for that. If it doesn't remove enough you can then go to more intensive treatments like rubbing compounds.

Try a little Bar Keepers Friend in a out of the way place.

If it's just the filon siding turning yellow then there's nothing that will help.
That's the reason my next RV will have full body paint.

It probably needs a lot of polishing compound to bring it back to white followed by a good wax job. There is no easy way around it.