Our 2008 5er came from the factory with a MorRyde pin box, and MorRyde suspension.
I'd be speaking with the selling dealer, if they have proof of what pin box was on the trailer, that should settle things quickly.
update: From what I can find on the internet, it did not come with a MorRyde hitch.
On the other hand, the Moss-Magnuson act says the mfgr must prove the aftermarket hitch caused the damage. I would let MorRyde in on this, they just might help fight your case.
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2000 Winnebago Ultimate Freedom USQ40JD, ISC 8.3 Cummins 350, Spartan MM Chassis. USA 1SG, retired;PPA,Good Sam Life member,FMCA.
"We the people are the rightful masters of both the Congress and the Courts - not to overthrow the Constitution but to overthrow men who pervert the Constitution." Abraham Lincoln