You can go either way with 3/4 ton truck. You could go with a light 5er or a heavy TT.
Fuel economy is going to suffer all around but more so with 12K 5er. You are losing economy by weight, height, and fuel. When you are pulling weight and drag (tall) up mountains and at altitude gasser produce their towing capacity at higher RPMs. That means more fuel. Of course the question there is Cents per mile, not necessarily MPG.
The truth is a diesels have the advantage as far as towing goes. A good gasser can do well, but diesels are still better, but cost more.
We had a tall toy hauler (closer to a 5er for drag) and besides needing to be cautious around trees it inhaled fuel. The diesel had little trouble hauling it around, but frugal on fuel it was not.
Grand Design Imagine makes a nifty TT called the 3150. Fully loaded it tops out around 9,000. That will still make a bite in the fuel but 3/4 ton should have no problem with it. It also has a 5er sized front hold and is nice complete with walkin closet.
