Rubbing Compounds, waxes...pads....

I remove oxidation in painted surfaces by doing this: 1/2 gal. of Jasco Prepaint Conditioner mixed with 1/2 gallon of water in a garden sprayer. Spray surface and brush with a soft bristle brush. Spray solution a second time and immediately rinse, using the same brush to agitate lightly.

For very soiled surfaces like boat hulls encrusted with dried algae, I'll sometimes break out the phosphoric acid and use a dilute solution of that along with a pressure washer.

I have buffers and various rubbing compounds. If wishing to use a buffer, I would suggest to use 3M ultra-fine rubbing compound. There is little to be gained and more potential risk using coarser compound.

Barkeepers friend is good stuff but expensive for the job. It also works well on brass.

For badly oxidised gelcoat best to use a compound intended to remove the oxidation. 3M has 3 grades of compound. The most abrasive is very gritty. Not sure if I'd use that on thin RV gelcoats. Maybe use the medium, then the fine. Finish off with a good sealer or wax such as Meguires Flagship. Without protection that hard earned shine will disappear in no time at all.

I had often read about using Barkeepers Friend on an RV and and waxing with Zep floor finish.

Not being one to trust what is on the net without some research I found that Barkeepers Friend is a mild abrasive with oxalic acid, neither of which should harm anything on a fiberglass RV. Removes oxidation and so far all stains.

Wear gloves! I follow directions and apply with a sponge and the results are great, even leaves my 23 year old caulk looking fresh and clean. Take extra care rinsing, the residue does not want to come off. It is a bit of work so I am only doing one side at a time.

What is the best remover of rain-borne dirt embedded in the paint?

I do a 3 step method for my front cap.
Presta Gel Coat compound using a DA buffer with a wool pad.
3M Finesse It 2 Finishing Material with a white foam compounding pad.
Collinite #845 with a black finishing pad. Or NuFinish.

Here are some old posts dealing with Oxidation Removers.

Is it a DA buffer?

My previous RV had oxidation and I found Meguiar's Heavy Duty Marine Oxidation Remover was the best and left an amazing shine. It worked well by hand/terry cloths or polisher. Polisher pad or terry cloth have to be cleaned often as they loaded up with a lot of oxide. If its bad, it may have to be done more than once. Then wax after.
http://www.meguiars.com/en/marine/products/m4916-heavy-duty-oxidation-remover/

I have a 31' fiver I have just working on. Think 2 years MIGHT finish it.
I have tried several things, the best being so far, a SOS pad and Protect All Oxidation Remover. Lots and lots of elbow grease and a buffer, too