Remove Rust & Prevent Future Corrosion:
a) wire wheel rust off, with drill
b) use Extend to convert rust, into a phosphate (Auto Zone stocks it, in small can)...sealing it
c) Metal Prime / oil base
d) Metal Paint, flat black
e) spray with Fluid Film, even inside of receiver square tubing (Auto Zone stocks it, in spray can)
If you use your hitch it will get metal/metal hits and lose paint. Swapping out receivers, hooking up to trailers, etc.
It will rust.
If you can't handle the sight of the rust, pick a brand and color (Rustoleum flat black, for example) and buy a few cans. Sand off the rust, then lightly sand the rest of the receiver and paint it your preferred color. Then every time you ding it inserting a hitch all you have to do is grab a can and re-spray.
I agree that it's an important step, although I use Ospho because I've always had a gallon around. It's the same thing, phosphoric acid, and it stops rust and turns it black. I did a 17-year-old rusty 5'x8' trailer the summer of '15 and painted it top and bottom with rattle can red Rustoleum primer. It stays outside and still looks great, even the mesh floor, etc. No bubbles, nothing.
Somebody call that guy that started a thread about concerns of surface rust on welds of his new 4Runner. Obviously you need to go to the dealer and demand that the warranty cover this since you spent good money on this truck. Maybe make a claim to Toyota corporate suggesting a recall.
Or just get some Rustoleum rusty metal primer and topcoat with satin black.