Well, so far I haven't seen any one say they're a structural steel engineer or something related to designing steel frames for trucks or MHs, or did I miss it someplace?
If one pops up, my question would be "how do they determine the weight capacity that they post"? My guess would be they assign a weight that is very, very safe/conservative because they have no idea what people are actually going to do with the hitch. It's the mfg or engineering firm that stands a lot more of a chance at being sued if one fails.
I'll bet there are people out there that put put more excess strain on a hitch like pulling 4X4s out of the mud (it was only a 3,500# jeep and the hitch is rated for 3,500#) than excess strain that the OP might put on the hitch.
Be safe - remember that the next time you coast off of a hill 2 MPH over the speed limit, you're just as illegal as the OP could be. We'll maybe you're more illegal since 1 MPH over the speed limit is against the written law. I can't say I've seen a law that specifically says you can't tow a few pounds over a hitch rating. They might use a law on unsafe conditions, but that's for the legal people to interpret. Might be that if it wound up in court and they go back to the designing engineer, he might say that "Oh,for that purpose it was well within design limits".
Bill