One time I had an old aluminum-sided truck camper with chalky, dull, flat-looking white paint. I used Comet powder cleaner and a sponge and washed it, came out looking very shiny and clean. Just as soon as it was dry I waxed it with -of all things- Simonize paste wax. Don't know how long that shine lasted, a month or so later a guy drove in and bought it, leaving me without a camper for the rest of the summer.
Look at it this way, if that does not work it will at least be clean enough to paint successfully(unless you wax it).
2000 Winnebago Ultimate Freedom USQ40JD
, ISC 8.3 Cummins 350, Spartan MM Chassis. USA 1SG, retired;PPA,Good Sam Life member."We the people are the rightful masters of both the Congress and the Courts - not to overthrow the Constitution but to overthrow men who pervert theConstitution. "Abraham Lincoln"