Bird dung has uric acid in it. Uric acid eats the clear coat finish and allows the organic ingredients of the dung to stain the underlying color layers of the paint finish. Once this layer is stained, little can be done to remove the stain. You might try bleach to lighten the stain, but if the bleach concentration is too strong, it may also remove the color and leave a "blotched area" on the trailer.
There are a few "bird stain removal compounds" on the market. Few of them, from what I've read, work as advertised.
If you're lucky and if the stain has not penetrated the entire color layers, you might be able to use an automotive compounding product to "remove the stained layers". If you do that, be cautious not to remove too much of the color layer or you'll be down to "raw fiberglass". If you use a compounding product, expect that you'll likely destroy the surrounding clear coat layer and the entire area will probably be subject to a dull finish once the wax wears away, so you'll need to keep it sealed/waxed to protect the underlying areas where the clear coat is removed.
To my knowledge, there is no good solution short of removing the stained paint layers and repainting.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS