Toy haulers have a heavy tongue weight by design...when you load the heavy toys in the garage behind the axle, it will actually lighten the tongue weight. What the loaded tongue weight winds up at is a function of what you load in the garage and what you load in storage up front, as well as where the fresh water and gasoline tanks are located...since most TH users boondock out where they can use the toys, plan on traveling with fresh tank full and fuel tank full.
That said, my guess is that when it's all said and done you'll be over your payload limits on the 150. The Roadmaster helps stability and the air bags will help level the rig, but neither do anything to increase your rear axle rating or rear tire ratings. Personally I would be OK pushing the GVWR, but once you go over axle/tire ratings you create safety issues.
Whether it's "safe" or not depends on what the whole shebang rolls over the scales at...you might be OK but if I had to guess I would say no. If you plan on carrying passengers and/or stuff in the truck bed when towing, I would say definitely no.
The 350 would be a much better choice.