Cargo Carrier Hitch sizing?

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.