I'm sure any automotive type car wash soap would be just fine. The main idea of the Mr. Clean thing is that you don't have to hand-dry the car/trailer with a towel to prevent spots. It seems to work well for me, but as you noted, others have had mixed results. they do offer a money-back guarantee, though. Anyway, I bet the reason for these varied results has to do with the quality of the water going in. it can only treat it so much; if your water is really bad to start with, it may not be able to make enough of a difference to prevent spotting. All I can tell you for sure is that if I wash a car at my house, and don't towel-dry it, it'll have spots. If I wash it w/ the mr. clean thing, no spots.
The clear-coat on your trailer will prevent the alluminum from oxidizing. Alluminum only oxidizes if it is exposed to air; the clear-coat seals the alluminum surface, so no air gets to it. Wax protects the clear-coat finish. There have been alot of problems with late model airstreams with the clear coat sticking to the alluminum. The only thing they say you can do to protect it is to keep it waxed with "walbernize". But people have reported that they've had peeling clear-coat even though they've walbernized religiously.
"Walbernize" is a liquid wax that airstream recommends. its very easy to apply and remove; not like a paste car wax. just wipe it on, let it haze over, then a light buffing w/ a soft cloth. very little elbow-grease required. couple times a year...maybe more, since you're down south.
'73 Safari 23'
'00 Dodge Ram 1500 4x4 QC