Unfortunately, the best trailer hitch in the world cannot give you the higher capacity cooling system, “beefier” final drive ratio, transmission cooler, and other vital items that are part of the manufacturer’s towing package.
The manufacturers of the trailer want to sell you a trailer, come what may. And, your mechanic is ill-informed, unfortunately. I predict problems if you try to pull this trailer with your “non-towing package equipped” Montana.
If you want to see an example of what I am talking about–albeit on a different vehicle–take a look at:
http://community.cartalk.com/posts/list/2125405.page