Follow-up to my original post:
I've had our trailer home over the past week to attend to the yellowing. As someone stated earlier in a reply, it happened gradually over the years and didn't really look bad except it wasn't shiny white. Nothing was chalky at all and it wasn't dull, just not bright white.
I like to patronize the locally-owned marine dealer when I can so I went to him again for the products I used. He sold me a stripper to get rid of any old wax, a polish to work off the yellow & get a good shine and the wax to protect it.
I worked sections separately from start to finish, doing the roof first, then one side and the back in another stage and finally the other side and front last. Luckily the weather has been cool and I've had shade from my trees by the afternoons for waxing.
Mission accomplished: we have a nice white trailer again.