Loaded, we're right at 19K with our 3950 without water. My 2011 Silverado 3500HD dually is a good match. The truck is rated at 21,100lbs. It's the only one of the 2011's rated for that weight.
My previous, a 39' Victory Lane was a good match for my '04 Ram 3500 dually, but after white knuckling the 3950 with the Ram for 3500 miles, I had to upgrade the truck.
Personally, if I had a 2012 that was rated for 21K (you didn't say what you have) I'd try it to see how it works out for your style of towing. But like me, be ready to change TV if your 3990 is too much.
Only YOU can be the judge of what is "too much" - some will tell you to go to a Class 8 hauler and others will tell you a 3/4 ton is fine.