POR-15 might be an option but if it were me, I'd use a rattle-can or brush-on rust converter after brushing off loose paint and removing dirt & grease. Best would be to re & re the receiver of course.
Had a look under the trailer? You may or may not find it could do with some touch-up too. Our TT got delivered in the winter and the salt was not nice to some of the frame and various exposed metal. Need to get under there one day and will brush on some Tremclad (sold in Canada by Rustoleum).