Yeah, the cruise control can't anticipate, it doesn't know what's half a mile up the road. Adjustments aren't made until it's too late, so you wind up losing much of your inertial speed to gravity and then it requires a significant amount of fuel and horsepower to overcome just the gravity of the hill, much less to get the truck back up to speed on top of that. Then, it's too sloe to react coming back down the hill, and you burn too much fuel when starting down instead of letting gravity do most of the work.
A cruise control on a car, van or light truck in hilly areas is a significant hit on fuel economy, and it's merely magnified with a heavier vehicle.
As for the gadgets, some work, some don't. Gadgets that work do so by overcoming something specific that hinders better fuel mileage. And if each one works, then each one will add a percentage to your economy. If you get 8MPG and add something that gives 5%, then you now get 8.4MPG. If something else adds 5% to that, then it's 8.4 x 5% for 8.82. Then, it's 5% times 8.82 for 9.261, and so on, so that it's not really 6 times 5% (30%) times 8 for 10.2 MPG, it's more than that. It's a grand total of 10.72MPG (which is 34% of that 8MPG).
Airtabs, for example, work, because of simple aerodynamics in reducing the lower atmospheric pressure immediately behind the vehicle which produces "drag". If you do the other 5 gadgets that reduce fuel consumption, that drag is still gonna be there, and when you add the Airtabs they reduce the drag and add to the fuel economy savings that the other gadgets produced.
If you do all those gadgets and you're now at 10.72 MPG, and then add nitrogen, you'll get yet another 2% or so boost on top of whatever fuel mileage you are then getting (the more tires you have the better the boost with nitrogen). So, yeah, they all add up.
Slow and steady, even in expediting, wins the race - Aesop
