Actually, power is probably the least important factor when determining the towing capacity of a vehicle. Most important are brakes & supension/ frame/tires and hitch ratings.
I've had my CDL for quite some time and have hundreds of thousands of miles seat time in bigger trucks so maybe this example will show you what I mean:
My first over CDL truck was a 1979 IH1700 10' dumptruck. It had a GCWR of 60,000lbs and a GVWR of 30,000lbs. The engine was a 345 gas V-8 with only 185HP. It had less horepower and torque than my 1991 F-350 pickup. In fact it had less horsepower than my wife's minivan! The difference was the brakes and frame/suspension. Massive in that department, it could stop the 15 ton tag and backhoe I towed behind it. It did the job and stopped the trailer safely.
To answer the original question, I would think you'd be fine towing a 10K trailer behind a dually 30 series stakebody. It will be slow as molasses behind a 6.2L, but it will get the job done. Just try to find out the GVWR from GM. Shouldn't be a problem.
One area of deception by GM, Ford & DC you need to watch out for is MAX tow ratings: most of these ratings are 5TH WHEEL ratings, not conventional tow ratings. The 5th wheel ratings are of course much higher, since the load is moved over the rear axle, not behind it. A lot of guys will tell you their truck "can tow 15,000 lbs", but that's 5th wheel. The conventional ratings are thousands lower. So when you get your GCWR number back from GM, that doesn't mean you can tow the same load off the back of the truck as you can with a 5th wheel.
Make sure you have a truck shop attach the proper hitch to your truck and make sure they certify the truck to tow the load you specify to them. You'll notice most body companies will affix a small decal next to the hitch that will show max TOW capacity and max PIN weight. That's what the MVEC will look for when they stop you. Also make sure you have a med card (required when operating a combination of vehicles over 17,000lbs) and make sure your registration has enough weight on it to cover the truck & trailer. If your trailer is under 10,000lbs, most states don't require you to register your combination weight on your registration.
The company that adds the secondary truck body to the chassis determines the final GVWR of the truck. The GCWR is determined by the manufacturer of the vehicle. With the exception of adding an airlift axle or additional suspension components, no secondary body company can re-rate a truck to have a greater GCWR than the original manufacturer, which in this case is GM.
Just because the military originally spec'ed the vehicle does not mean it can tow a lot of weight. the military may have built that truck simply to be a stakebody that wasn't used to tow a trailer.
I deal with this stuff on a weekly basis and have MVCE cops as friends. They tell me what they look for, so ask away and i can try to help you avoid the pitfalls.
