Any experience with new gas engines hauling heavy??

I pull 12.5K nearly full time with my V10, and occasionally as much as 16,000 all on mountainous terrain. I think he'll be fine.

Also, keep in mind that the vast majority of the motorhome market has the Ford V10, with combined weights up to 30,000 pounds. If he wants a really nice gas-powertrain ride, you can spec an F650 for up to 37,000 combined, and gain nice features like air-ride seats, double saddle fuel tanks etc, for not much more than an F350. Just gotta add your own tow-bed.