I recently had to replace my 10 year old trailer that had severe internal corrosion even after religiously washing down after every trip and even squirting down the drain holes in the cross members as much as I was prepared to do as there were many holes to do.
The new trailer has twice as many roller and crossmembers to rinse so I got the old "Grey cells" to work and integrated a simple garden irrigation system into the rinsing system. It works so good that I just have to share ($31 off Temu)

Follow link to video of it in action. https://www.facebook.com/reel/1964832670746497




I've since run a flat irrigation hose up and down the outside arms of the trailer - 15m of it for $8 from the Warehouse. Now it's all automatically rinsing whilst I sit and have a beer. 

Link is fixed now.

I have a mate who did a similar thing Steve. But he had a 20 litre tank of freshwater with a pump mounted on his trailer. He wired it so that when he launched the boat he plugged it to a connector and thepump started up once he restarted his vehicle.