Until recently I towed our 2008 Avalanche behind our American Tradition with the ISL Cummins rated at 400 HP. No problem. I did not notice any big change in fuel mileage either. Now towing a 2008 Hummer H3, but that decision was NOT based on curb weight. If I owned an Avalanche and wanted to tow it I would do it again.
I assume you have the C7 Cat engine rated at 330 HP, which I have previously owned in a 39 foot motorhome (Discovery). I do not think you would notice much, if any difference between the two vehicle weights that you mentioned. It's only 1700 pounds. I would definitely not let this difference force me to make an otherwise unwanted change in dinghy. Additionally and perhaps more importantly, my C7 powered coach had a better horsepower to weight ratio than my current coach with its larger engine, because as the engines get bigger they are mated with heavier coaches, until you get, perhaps, to the top end of the engine/motorhome combinations (500 to 600 HP) which neither one of us have.
Paste the link below into your web browser, fill out the Fleetwood form and you can download the brochures for the correct year and model of the two coaches you are looking for:
That was you can study the two models and any differences that may exist.
As many will post, there are higher end coaches out there and only you can evaluate that. We have owned a 2005 Fleetwood Discovery 39S (FL chasis)and we loved it, and only traded it for a newer, larger American Coach Tradition with tag axle (Spartan chasis).
If I weere picking between these two I would pick the one that looks better and has the floorplan more suitable to my needs. I know the Providence was a model 39S but I don't believe you ever quoted the model of the Discovery. They are esentially sisters with the Providence supposedly the next step up on their food chain, so to speak.
Good luck and enjoy the hunt.