I tow a 24' and a 28' featherlite enclosed trailer (not both at the same time) behind my 39' Winnebago DP all over the country.
I have the 330HP Cat motor - it's a little bigger than the ISB but smaller than all the other Cummins engines and I have NO issues with power.
The thing to look at is what transmission you have. The Allison 3000 is the big one and the lighter DP's has the Allison 2100 or 2500 which is where you will have a 5000lb tow limit.
Also have your hitch checked - as mine was rated for 10,000 lbs, but the tongue weight was only 500lbs. I got a new super heavy duty hitch built and I usually run a 9800lb trailer with about 1500lbs of tongue weight.
I use both an AirSafe air bag hitch and also weight distribution bars which makes a WORLD of difference in the tow quality. People think that the air suspension will compensate, which it does, but the ride quality is night and day with the weight distribution hitch.
The AirSafe hitch makes it feel like I'm not even towing a trailer.