I agree also with what has been said. A couple of small trips to get a feel build the confidence and then the big one. Believe me you will enjoy it. Nothing like a new TT.
I had a 24ft Wilderness that I was towing before and I figured it was around 5000lbs loaded (no water) and I had no trouble with my 4.7, better than my 'ol 5.7 Suburban FOR SURE. Maybe have a few gallons but I would travel empty if you know that where you are going has water. I understand what you and the bride say about that jack knife couch. That was my reason for going to a different trailer.
We go through the mountains here in Alberta and all you do is take your time and depending how much you actually load in the trailer, payload in the truck, a good brake controller, WDH and anti-sway bars you should not have any trouble. Did you happen to check the actual sticker dry weight and tongue weight in the trailer? Possibly once you get the trailer, do you homework on weights etc, get your whole rig (TV & TT) CAT scaled weighed to give you the reassurance.
Something that may also help is picking up the Trucker, RV and Motorhome Driver guides for your neck of the country and heading westward. I purchased it from Trailer Life I believe but the website is
Welcome to Mountain Directory. These can help you plan which areas i.e. mountain grades to avoid (white knuckle experiences), lengths of certain grades that kind of stuff. You may find it helpful in planning your trip.
Good luck.