You need to check with Newmar to see if the chassis will tow anymore than the 5,000#'s as it's NOT just the hitch that might be the limiting factor. Could be the transmission, rear axle, brakes etc. What ever is the lowest rated component will be the one that controls. Only Newmar has the right to say what you can tow due to Federal law.
2009 45' Magna 630 w/Cummins ISX 650 HP/1950 Lbs Ft
Charter Good Sam Lifetime Member, FMCA, SKP
RV'ing since 1957, NRA Benefactor Life, towing '05 Odyssey