Some of the small 1/2 ton SUVs & pickups coming out of Detroit now days have a tow rating with & without an equalizer. I fell for it & bought a Nitro a couple years ago. Rated for 3500 lbs without & 5000 lbs with an equalizer. Then paid $300 for the equalizer. Then test drove the truck with the boat in tow with & without the equalizer.
So the bottom line is that it does change things. Rather than the truck squatting in the rear, the equalizer pushes the weight to the front of the truck. Makes the whole thing sit level. Makes the truck steer & brake a lot better. Might even take out the "tail wagging the dog" problems with a single axle trailer, but I have a tandem so that was never an issue.
Save on maintenance? Yea, probably will make your shocks & springs last longer. Especially the rear.
But improve gas mileage?? That seems like a stretch to me. Your still driving an underpowered vehicle with the gas pedal on the floor to go up a hill, no matter which end of the truck is weighted down.
Hooking up the rig is not all that easy either. It's just a change in the process, but it makes it twice as long & you'll love all the grease getting all over your board shorts & Reefs.
In the end I decided I'd rather have a bigger truck that is better able to haul the 5k lb load on it's own. So I sold the Nitro & bought an Avalanche. The rear coils on the Av are a little soft, so if I'm towing a long distance &/or have a lot of weight in the truck or boat, then it gets a little bouncy. So I installed a $100 set of Firestone air bags in the coils. So now I pump a little air in the bags & the truck rides great, even loaded up.