Over time, UV radiation (sunlight) will damage or destroy most plastics. There are some newer plastics that are specially formulated to withstand UV Rad, but I'd guess that very little of that is going to be found in our trailers!
Take a look at 1990 and later cars; how many have plastic headlight lenses that are fogged and yellowed beyond use? I know, because I watch for them - the answer is A LOT!.
There are some remedies if your plastic (both transparent and opaque) isn't burned completely up:
1) You can use Rubbing or Polishing compound as a light
abrasive to cut the surface damage off opaque
plastic. Although Polishing compound is a finer grit, either item should clean your yellowed plastic and leave a nice finish.
2) I use a 4" linen buffing wheel chucked into a drill to polish both opaque and
transparent plastic back to original condition. This method is a little more expensive (under $10 without the drill) but the effort is much less and the results are better.
To use a buffing wheel, you should
use more than one type of abrasive. You can find both buffing wheels and tubes (or sticks) of abrasive compound at nearly any hardware store and most auto parts stores. For plastic, I use 'Tripoli' (fine grit) and 'Rouge' (extremely fine grit) to polish out foggy, yellowed, or scratched areas.
You should use only one type of grit per wheel if you can; otherwise, hold a flat metal blade - like the back edge of a dinner knife - perpendicular to the wheel (like you are buffing the slim back edge of the knife). This will force most of the grit out of the wheel and you can then apply a finer grit.
With a 4" linen buffing wheel priced at about $4US, it's much better to buy a wheel for each grit so as not to contaminate them.
If you're lucky, the store display will list each type of grit and its' uses. If not, here's a basic list:
WHITE compound is Aluminum Oxide - fairly fine, but hard, and is used primarily for metal. I polish rough stainless steel to a chrome-like finish with this. Perfect for aluminum; follow up with a good coat of wax and your metal window frames will look like new!
BLUE compound is known as Tripoli - is fine, and is used as an intermediate metal polish, or for 1st cuts on plastic. It will leave opaque plastic with a beautiful finsih, but it will leave transparent plastic with a slightly foggy look.
RED compound is called Rouge (but all buffing compounds are also collectively called 'rouge' so don't get confused!) and is extremely fine. Use it for final finishes on metal where you want a mirror finish, and on transparent plastics to leave them perfectly clear.
TO PROTECT your newly-restored plastic, I recommend 303 Aerospace Protectant. Inexpensive, in the auto polish aisle at Wally-World. Some people prefer Armor All, but I like 303 much better.
An added benefit is that you can also polish out those foggy yellow headlights and save the $145+ replacement cost!
DISCLAIMER: I will not be held responsible for the time you waste polishing everything in your house, car, and trailer. Once you get started with this stuff, it's hard to stop!
Here's a link to my Crown Vic Car site. Page through the pics and look at the crystal-clear headlights. When I bought the car, you couldn't see the lightbulbs though the fog & haze!