On your plastic parts you can use diluted Awesome, from the dollar store. It's quite powerful and will get those plastic parts white again. HOWEVER, do not let it run down the side of your rig, it will etch fiberglass or gel coat. I use the blue painters tape and tape some paper towels or those blue paper work towels underneath whatever I'm working on.
I've not heard of gel coat yellowing unless the rig is really old, no clue on that
Jan & Thomas
2012 Drv Mobile Suite
2012 Ford F350 Super Duty