Cargo Carrier Hitch sizing?

[GaryKH]

I've recently bought a second hand Thule Terrapin 684 cargo carrier. As my van is parked, with no insurance for the winter, we drove down to pick it up in my FJ, with a 2" receiver. It fit, no problem. When we got home & tried to plug it into the 2" hitch receiver on my '96 Roadtrek, it wouldn't fit. I double checked with a 2" hitch that it fit into the RT's receiver & that it was the same dimensions as the carrier, it was. Strange!
The only thing i can think of is that the corners of the 2" channel were slightly rounded on the hitch, but not on the carrier. Would grinding the corners down a tad weaken the whole thing or be ok?
Also, does anyone know why these were discontinued? They seem to be a good product.

[GaryKH]

Thanks for the tip on the rust. I'll check it out later.

[SplashDancer]

Maybe just some small amount of rust in the receiver on the MH, as I've had that on my '04 Avalanche (wouldn't fit till "cleaned out" vs a '13 truck I also have. I actually sandblasted the inside of the hitch receiver on the Av, then it worked, although was still tighter than the newer truck's, ??