Yellowing Skylight

Wet sand with very fine paper and polish with plastic polish. This is the same process that the auto kits offer.

It may be that the plastic has yellowed throughout but the process to restore is cheap and doesn't take much time---worth the effort.

Not likely, its UV damage and the plastic is becoming very brittle. If it is the 14inch square, it would probably be cheaper and less work just to replace it, Walmart and any RV store have the replacements in stock, just check the hinge type (there are three different styles).
After that, it would recommend putting on a second cover, Maxair or similar, in addition to providing double protection, it allows ventilation, even in the rain.

Ken

Krylon Fusion Paint for plastic.

I painted all the trim outside and inside on an older TT for a friend, made the whole RV look like brand new.

I even painted the side mirrors, porch light, stove vent, fridge vent that had turned yellow on my MH and it looks like I bought new ones. AND it LASTS and LASTS. Amazing paint.

This is the fixed skylight over the tub/shower. Structurally, it still looks solid, just discolored.

How about trying one of those headlight restore kits? I used one on old headlights and it did a pretty good job of clearing up the lenses. I don't see why it wouldn't work on a skylight.

That's exactly what I did. Ultimately, it cleaned off the haze, but the yellow remained, which may be unavoidable. I can live with that. Thanks everyone.

Since we had a crack develop in ours over the winter, we just replaced ours. Both pieces were very cheap from the manufacturer (under $15 each inside and out). It's amazing how much brighter it is in the bathroom. I didn't realize how dark it had become. Well worth replacing, and not all time consuming either.

These dome are made with some styrenic and you cannot bring them back white or clear.
The car headlight are polycarbonate or acrylic and the UV only affect them on surface, it's not the case for styrenic product.