There is 10,000lb capacity on diesels, but two things. the hitch rating AND the tranmission. The lower cost diesel motorhomes USUALLY have the Allison 2000 or 2500 transmission, and the beefy ones have the Allison 3000 transmission. This is a big piece in the tow capacity.
I have an allison 3000 and can safely tow 10,000 all day long and I have done it for over 40,000 miles. I did have a custom hitch made that was much stronger than the OEM hitch, but that is another discussion and is related to my wanting things to be more than safe.
good luck on your search!