I've been doing a ton of research and am looking at similar coaches as you.
I also have a 3/4 ton Duramax CCSB 4x4 I tow a 5th wheel with, around 21k combined currently.
My wife and I are also considering the Journey 34Y, not the 39N that you are interested in.
I've driven the 34Y 2009 and with this coach, after all my research and driving it, I am confident it will tow my 7k lb toad dmax reasonably well.
The wet weight of the 34Y is 24,200k roughly.
Here is what I believe to be the most important info for you to find out.
The 2009 Journey is the 39Z. I strongly recommend considering this year coach as it comes with the Allison 3000 and is truely rated to tow 10k lbs.
You will have much more CCC on this model.
The new 2010 39N (not Z) (and this applies to the 34Y as well) comes with the Allison 2500, and although it has a 10k rated hitch, if your at your max GVWR, then you can safely only tow 5k. I have a feeling these 39 models are going to be a bit heavier which in turn means you'll be sniffing the max GVWR before you even hook up your 4k lb toad. Sure the coach will handle it, but I'm a bit fan of having at least a somewhat overbuilt coach, vs. being close to maxed out which I feel you will be on the 2010 39N with that tranny.
Great rigs, the quality is really there, a lot I like about them. I really want to be under 36ft end to end. Considering a Tiffin Phaeton 36, but I have heard they are 39 ft end to end so that may be ruled out.
Hope this helps...happy hunting