I agree with BigZ here. Don't exceed the GVWR of the vehicle. However, now that I got that little disclaimer out of the way, I'd make absolutely sure that it was loaded appropriately. Load the rig and then make a trip around the block and down the highway. If you aren't sure about it, don't do it. Make sure to fill it with fuel and water after you get closer to your destination (cuts down on weight).
I pull a 4000# trailer on long trips to Idaho , and I'll tell you, it takes every pony to keep it moving. The terrain around here is uneven and steep, especially in Western Oregon. Keeping highway speed is difficult. Don't plan on being in a hurry. Also, with a tandem axel(or triple axel at 9000#) boat trailer the surge brakes should work great, and the H2 has 4 wheel disk brakes that work great with stock size tires/wheels. I've noticed, in hot summer months, the transmission gets very hot, and it shifts into 2nd gear (incessantly) getting up hills. One other thing about towing with a Hummer, it will learn your driving habits on long trips, and it does help.(a little) The programmer is a nice doo-dad, and I'm thinking of picking one up as well.
It feels more stable than an Excursion, if you can believe it, but it definitely lacks the power. However, what the H2 lacks in power, it makes up for in GOOD LOOKS.
Do you have air suspension? This would be the most valuable asset to towing that load with your rig.
Anyway, these are my experiences with my Hummer, and I hope they help you with your trip. To be perfectly honest, I probably wouldn't do that with an H2. I'd use my
Ford. To all you GM lovers out there, I just had to drop the
F bomb.
Good Luck and Have Fun!