There are several threads on refinishing plastic and fiberglass components. Some people have good luck with various kinds of paint specific for plastic and fiberglass. Others have performed more of a chemical conversion to remove the yellow layer and turn it white again.
Do some searching on Air Forums and on the web. Lots of information is out there regarding freshening up plastic and fiberglass.
We're letting our shower (just like yours except for wallpaper) age naturally. Hey, it's an old trailer.
Lucius and Danielle
1992 29' Excella Classic / 2010 Interstate
2005 Chevrolet Suburban K2500 8.1L
2018 GMC Sierra K1500 SLT, 6.2L, Max Trailering
Got a cooped-up feeling, gotta get out of town, got those Airstream campin' blues...