Of course I'm biased, but for more than 5 passengers, I can't imagine a better match for the 9000-pound trailer than the "classic" Burb K2500 with the 8.1L and 4.10 axle. We've had ours for 11+ years now and have been thrilled with it all along the way. It's actually not a bad hauler, either, as I can get about 20 sheets of 4x8 plywood in it with the seats down and still close the tailgate.
Our Burb is rated for towing 12,000 pounds with a 150-pound driver and a full tank of gas. It has a 6000-pound rear axle and a payload of over 3000 pounds. True, the tranny is a 4-speed, but I've towed all the mountains of the Pacific Northwest and the Rockies and have never had a problem keeping all our stuff up to speed. As long as I keep it at 60 mph (in Tow-Haul mode) and I'm not going up anything steeper than a common overpass, it stays in overdrive and at a tad under 2100 RPM.
Interestingly, your situation is the opposite of ours. The number of passengers going along on our trips is reducing as our kids reach college age, and we're actually looking a 3500HD Diesels right now because DW would like to move to a fifth wheel when it's mainly just the two of us.
Shortly after we purchased our Burb, one of the concerns I had was that the 8.1L engine would be discontinued, so I was extremely glad I got it when we did. It's not a great commuter vehicle due to the horrible fuel mileage running cold, but for towing it's a very comfortable and powerful beast. Since it's been used largely for towing and vacationing, it also has only 72k miles on it, which may delay how soon we move over to the diesel pickup - unless we find someone who wants the Burb more than we do.