Had three 5.9 and on my third 6.7. All have been in a Ram p/u except this one which is in our new 2103 Fleetwood Expedition 40x . Towing the toad we are at around 32500 combined. It is rated at 360 on this model with 800 or so of torque.
I've never had any power issues with any of them, and the MH is 42'. It pulls and drives just fine. We tow a Jeep Liberty at around 4500 lbs. I set it on economy on the trans and set the cruise and there isn't an issue. It hardly ever downshifts on the hills.
The Fleetwood Discovery is rated at 380 hspwr and 1000 on the torgue. I don't know how much bigger you need to go before a different engine is needed. I probably don't have the money for one of those MH's.