Don't know the first thing about Zep....never even heard of it until 3 minutes ago. That said, I would not use anything that's not considered a standard RV or auto product as long as the trailer has a warranty. You should find that info in the stack of paperwork. Anything out of the ordinary gives the siding manufacturer a nice clean out if they decide the product did any sort of damage, whether it did or not. An older/out of warranty trailer maybe.
APT.....DH thought he read on the 303 bottle that it only lasts 3 weeks. Is that correct? I believe I'm the decal protector only by default around here. Please tell me it's not every 3 weeks....I can't keep up NOW! The rear decal is the only one getting full sun, so if it's 3 weeks, at least that one may get it.