Most of the corrosion occurs whilst the trailer is at park after launching where that salt has many hours to do its thing. Once home then its washed off but the damage has already been done.

Why not just ad 2pac clear coating to the outside of the trailer. no more salt eating metal so to speak...

if the trailer is galvanized  & properly galvanized  inside the tubing &  painted on the outside  with 2pack paint this should prolong the life of the trailer , eventually over a long period of time the rust will start from the inside the tubing  , my idea of building a trailer is to have no holes drilled into the tubing to bolt brackets & whatever on  there are means & ways this could be avoided to drill holes into the tubing & still bolt things on at the ends of the tubing fabricate cover plates & bolt them on with sealed gasket after galvanizing pour engine oil in tubing  & bolt cover plate back on  apply a 2\3 coats epoxy paint with the oil in & galvanizing this should prolong the life span further

with oxygen in there it will rust with oil in will prevent \slow the rusting proses as yours has already started  if you good with a welder weld it closed 
when something is galvanized with the ends caped off [ closed ] holes are drilled for the galvanize to enter & exit   in the dipping proses

The Soak Hose sprinkler would work. I currently use two short bits of ordinary hose down each side then a T junction to a single bit with a hose connection. I put a good squirt of washing up liquid down the hose connector before connecting to the water. The soap foams up and cleans all sides of the internal tube. 

Externally, I use to spray inox on all the nuts, bolts and springs as well as the hand winch and moving parts at that end.
I use to do this once the boat was in the water as it seemed to almost remove the water keeping in mind that the movement up the ramp and to the parking area would shake off most of the water anyway

 I spray Lanocote (CRC product), on all sorts of things, including the trailer parts that show some tendency to rust. I like it, as it sets into a sticky coating that you can see and repels water and salt. The trailer is only 9 months old, so its too early to be sure, but it appears to be doing good. 

joker, i used lanocote on my trailer every third or fourth trip from new. after three years the galv was still shiny like new except for a few places. and it had one spot of grey under where the bung would drip. i reckon it was all the salt in the carpets dripping there. there was no signs of rust anywhere.  would have cost a couple hundred in lanocote overall but in my opinion very worth it. 

also good for sealing battery terminals and electric windlass/capstan motors etc

I have found a home grown mixture of used engine oil , fisholene, and shellac is wonderful for killing rust. The fisholene kills the rust, the engine oil penetrates (pulls the fisholene into crevasses, cracks and under the rust flakes) and the shellac dries and seals the fisholene so that water does not penetrate. This mixture is made up of equal parts of oil fisholene and shellac mixed together in a bottle and can be thinned with meths. Doesn't look too good but by gum it works a treat. I had a Austin J4 van on the waterfront where salt spray , sulphur, potash, phosphate blew on to it from port facilities.  We painted any rust with this and it stopped it in its tracks. After 3 years before we sold the van (they were very prone to rusting)  we water blasted the whole thing and it ended up like no rust. Shellac has to be made up from "powdered shellac " available from good paint wholesalers / shops. fill a far / tin with the powdered shellac and then tip meths in until the jar is full, screw lid on to stop the meths from evaporating, Leave for 24 hours until dissolved and then mix other ingredients in and off you go.