Best magnum MPG?

My old 73 dodge would get 17 consistently at 70 pulling a trailer. 360 two barrel and 3.23 gears. Hoping to drop in the stock 5.2 magnum and pull down 21 at 70. But I am gonna hop up the cam after I've got the cash the 5.2 in my dodge dakota would never get more than 15 mpg. I tried to granny it on the highway and it still got 15 mpg. So I floored it everywhere. Always got 15. Nothing more or less.