The best thing I have found to restore fiberglass gel coat is Meguiar's Fiberglass Restoration Kit
. It is available online, or from a marine supply shop if you happen to live near water.
The kit has an oxidation remover to get rid of the chalkiness, a polish to bring up the shine, and a wax for protection.
I used this kit on my fiberglass roof, and it took a surprisingly small amount of elbow grease.
If the bottom of your motorhome is painted, then you will possibly need to start with a polishing compound before waxing. I'm sure Meguiar's has something for badly-oxidized paint as well as for fiberglass, so it may pay to check out their "How To" section.