Grade, length of grade and altitude can really tweak on the ability of an engine. A 220 horsepower engine that got-by pulling 6000 lbs may fall flat on its face pulling 7500 lbs. It's a rare RVer who can resist stuffing even more clothes, food, toys, and miscellenous in a new rig. Solar panels, batteries, inverters, you name it. I would ask myself "did this engine have LOTS of extra power with the lighter trailer"? How many more "young" miles does your existing truck have left in it? New trucks have loads of goodies like exyreme power alternators HD radiators, perhaps more comfortable seats, more desireable cabin layout, to tempt. From what I've seen there is a lot of suspension difference between a standard 3/4 ton and "heavy" 3/4 ton.
350 cubic inches is not a real heavy duty engine.
In the end you need to sleep on all this. A new truck is expensive and only you have sense if the truck is perhaps borderline powerful. Not an easy decision IMHO. ๐ ๐ ๐