kaydeejay,
Your friend said it right that there's no problem stopping the rig in an emergency situation but not if a driver without any other braking help were to ride the brakes down a long steep grade and then expect to have the optimum braking ability from either the truck or the 5th wheel's wheel brakes. The OP stated he has a 2007 Chevy 2500HD D/A and therefore he has a helper called an engine brake which is standard for downhill braking help and there's no reason for him to even ride the brakes.
Yes, of course I do fully understand ABS systems and their good and bad as they are far from perfect on all surface, wet, dry, loose gravel, or even snow etc covers. They keep the vehicle going reasonably straight and at reasonably colse to the non-skid point until the trailer fishtails and become the boss.
A panic stop is not a series of applying the brakes or heating up the pads but rather a one time climb on the binders!
Did you ask him about the brake system on the 2007 GM 2500HD being the same OEM asm and component part numbers as the 3500HD SRW and the 3500HD DRW models? The 3500DRW version same brakes are rated to consistently and constantly stop an over 11,000 GVW. Check it out! Therefoore the OP's truck brakes are not an issue with 10.7K but his OEM tires likely are as he needs an adequate tire capacity and the larger tires give more square inches of tire footprint so he's kind of back to even on the score board.
I happy to leave it at that other than saying it's true he'd be better off having a 3500HD DRW truck for the 5th wheel he has but it's not a must.
Good fine point discussion Buddy!