Towing Elkmont 24' with Buick Enclave?

We bought an Elkmont 24 bunkhouse with sleeper sofa instead of queen bed. We tow with a 2000 Mercedes ML320 (6 cyl. SUV). Bought the Elkmont in TN and towed back to NC over the Smokies on I-40. No sway, no bucking but I noticed on return that the rear of the TV was several (3) inches below normal. We have towed a 26' boat for several years, from NY to FL, with no issues at all. I borrowed a digital car scale and measured the tongue weight of the Elkmont and it came in at 617 lbs. - what a surprise - about 200 lbs. more that I expected. The hitch on the TV is rated at 500 lbs. so I bought a weight distributing hitch. Subsequently the TV and TT are now level when towing but I don't notice much difference in the ride or towing.

I'm currently looking at upgrading to TV with greater capacity. May go with a new ML320 diesel or a truck (Toyota Tundra).