I have never seen a gas Class A Rv that is rated for over 5000lbs towing to begin with so it's no surprise your old rig was having issues, you are very possibly overweight pulling that toad with that rig. No matter what the rig, I would think about a lighter toad, you and the RV will be happier.
Outside of that, always go with the bigger engine if it's an option. Better to have it and not need it than not have it and wished you had. Sure wished I had ordered the 2nd air conditioner on the roof!