Fibre Glass finish is similar to that on boats (fibre glass and gelcoat).
The dullness and chalky surface is oxidization from UV Rays.
Wax is only covering the problem and not getting to the root of the issue.
You need to first apply a mild Compound to remove the oxidization, and then apply the wax.
Depending on how bad the oxidization is, will determine how heavy a Compound you will need to apply.
Best results come with the use of an electric buffer and a spray bottle (water).
After 25 years of boating, I can tell you waxing is never a fun task!
Hire an RV detailer @ $10.00/ft. and let them do the heavy work.