The GCWR for the 2500 ext van I drive at work is 15K lbs. On par with my 96 GM SW K3500 with a diesel of similar specs. This van is on par performance wise with that truck. The MB van does have a slightly lower gvwr, ie 8500 vs 9200. A 5500 GRAWR vs 6100 for the GM. I have had upwards of 8 55 gal drums, ~3500 lbs, of anitfreeze in it with out any issues. Van is 5400 empty wt.
I've also pulled a few smaller boats, granted only in the 2000-3000 lbs range. No issues.
I see no reason that this setup can not run down the road at its 15K gcwr and have issues frankly. Yes as noted, try to go up a 5% grade at 60 mph, you will not do this. It would take over 300HP to succeed at this. It will do 45-50 mph, maybe as low as 40 depending upon the road type etc at 15K.
I am also sure it more than meets specs for the new tow ratings. Then again, one has to remember, that has a low of 40 mph on a 5% grade to meet the new specs. It will do better. How much, again, depends upon multiple factors.
I get 21 running down the road at 70. 22 at 60-65, and over 23 sometimes low 24's when running on 55mph roads using cruise as much as I can.
Marty