+1 on the Oshpo.
Do not have to expend super amounts of effort removing rust.
It is very thin and gets deeply into the nooks and crannies. I have applied it with a brass brush 24 hours apart on badly rusted surfaces, sometimes the second attempt shows grey pitted steel where flaky rust existed previously.
Do not let it get wet or moisture to condense on it for 24 hours after application.
Paint sticks to it great and it must be painted
I am highly doubtful about rustoleum's ability to slow, treat, prevent rust.
Rust bullet and Por15 are $$$. Do not know how much better they are, but I suspect a couple hundred percent.
How much is a new trailer versus the time/effort and materials to slow the rust on what you have?
I'd inspect the worst, most structural parts first to see if they are structurally deficient and not worth the effort, unless you happen to have a welder and stock handy and the skills to re-enforce.