[travelin texans]

In your case with NO experience towing taking the salesman, probably with even less experience towing, along will only add to the confusion. You don't know what to expect towing while he/she is sitting along side telling what a good job your truck is doing, don't see this being much help to you.
Maybe your cousin could make you a helluva deal on a 3500 Duramax then you can go buy the rv of your choice. Or at least let him read all the info on how to determine real tow weights you've gotten here so he won't steer the next person, family or someone else, down the wrong path.