It doesn't appear that you have clearcoat...it would not have been original in '56 and highly unlikely a PO coated it. Besides, from the pics (thanks!) it is an overall even "gray" which is just oxidation...not milky white, uneven failing clearcoat.
If you have some Nuvite "C" you might try that. It seems that the "G", while coarser, breaks down faster so it may not cut through the oxidation as the finer "C" which stays effective longer may. We have had great success with "C".
Also, using too much polish "lubricates" more than "cuts"...teeny, tiny dabs is all you need. Keep at it, you'll get the feel for "what works" on your trailer...every one is slightly different.
Some folks are using Tripoli and a giant buffing wheel on a drill for the first pass...then switching over to the Nuvite products. That's what we are doing on our '56 which has more corrosion than our '64 did....but we have just done areas like window frames (which are not Alclad) and areas around lights & trim during our restoration, not entire panels yet. Also to knock the bumps of corrosion off, you may want to try a fine (we have used gray & green) Scotch Bright pads by hand...which will smooth them out but scratch, then work on getting the fine scratches out with the compounder. Again, be careful so as not to go through the Alclad layer...if you do that, there's no going back.
There are as many different techniques to polishing as there are polished trailers...some things "work" for some people & trailer conditions and others work for others. And somethings work on one panel of your trailer and other panels may need something different...the key is finding what works for you.
As for polishing being a lifetime commitment...it is, but only the first time is as much work as you are experiencing now. THIS time you are trying to get 52 years worth of oxidation off...next time it'll be only a couple of years worth. Our '64 has been somewhat neglected this year...it's been 4 years since we did our original polish, but it still looks good...next spring/summer, when we are finished with our '56 we'll freshen up the '64.