Looking over your posts again, I believe that 13.5 is still a bit on the low side, but it is within the range of what could be considered normal, depending on many other factors. I do believe it can be improved and I've seen better.
To explain some basics and since the truck is new to you, the Dakota was introduced as the first midsize truck and was intended to directly compete with small Toyotas, Ford Rangers, and Chevy S10 pick ups. As a midsize, it had as much capacity as a 1/2 ton full-size truck, which made it capable of carrying more cargo and haul more trailer weight than the competing smaller trucks.
The negative side was, the Dakota was heavier than competing models and therefore less fuel efficient. In fact, a fully loaded (with options) Dakota could weigh as much as a 1/2 ton full-size truck, and got about the same in fuel economy. The whole purpose of the compact truck segment, when introduced was to provide good fuel economy. As an example, an early model Ford Ranger 4x4 with a 2.3L 4 cylinder was rated to get about 24mpg, but to do it, it had to be as light as possible, which meant that it had to give up some of the capacity that made it a pick-up truck. The Dakota, on the other hand was intended to offer better fuel economy than a larger 1/2 ton while maintaining the same load capacity of a 1/2 ton. (Which typically exceeds the capacity of smaller trucks.) Sadly that strategy didn't last long, and the Dakota was discontinued, due to it's price and fuel economy. (Many customers came to the conclusion, why spend the money for a "small" truck, when for the same money they can enjoy a roomier full size??)
So what does this have to do with your Dakota?? Well, generally Dakotas are inherently thirstier. A V6 under the hood, doesn't guarantee better fuel economy. Those are general misconceptions. As an energy source, Gasoline only posses so much energy per volume, and it will always take a certain amount of energy to move mass. So say for example, it takes 50hp to push a Dakota down the road at 50mph, it won't matter too much if the engine is a 4 cylinder, V6 or V8. 50hp will require burning a given amount of gas. The only real advantage of a small engine is that they have reduced weight and friction, which can help save some fuel.
The main advantage of a larger engine is power. If for example it takes 300hp to pull a load, an engine that comes up short, (such as a 4cylinder or V6) will not be able to pull that load. A larger engine will also accelerate quicker and sometimes reach a higher top speed, but that means less fuel economy too.
It's possible that the "loss of power" you have noticed, isn't a "loss" but a "lack". You might be used to either a lighter vehicle or a more powerful vehicle, and to make up for it, you have to get on the throttle harder, to get back the "feel" of what you're accustomed to. As such, your fuel economy can be related more to your driving style than the truck itself. It's like driving a sports car for a number of years and then trading it in for a U-haul truck, but you still drive as if you're still in the sports car. Sadly, if fuel economy is a concern, you'll probably have to adapt to driving an entirely different vehicle than what you're accustomed to, even if you came out of another truck. The V6 isn't that peppy and to get it to go, you have to push the throttle harder. Anotherwords you're asking it for more power and the result is less fuel economy.
The smell if gas should still be a concern. You should never smell gas in the cabin. Being that this truck is new to you, have you checked to see if theres an air filter in the filter box? If it's missing, it can be the reason you smell gas. I'd still have the charcoal canister checked too.
I'll take your word that the tire size is normal. So going back to everything else, do the usual stuff. Tune up, check oil levels…BTW when you check the transmission ATF, make sure you're on level ground, the engine is warmed up and idling, the parking brake is set and the transmission is in neutral. You'll get a more accurate reading (Never check the ATF when the engine is cold, or not running, or the trans is in park.)
The only other way to increase fuel economy is driving style. Another-words, drive as smoothly as possible. You want to accelerate easy, not hard, like theres an egg under the gas pedal. Maintain speed as long as possible. If you see a light beginning to change, don't try to outrun it, if you see it changing from a distance, immediately begin coasting, and try to avoid using the brakes. You want to take as much advantage of the stored rolling energy you have to save gas. If you can time the lights so that you don't come to a complete stop, you'll save fuel because you don't have to accelerate mass from a standing start. (Acceleration is the most fuel consumptive part of driving).
When gas was $4/gal, I was using these techniques to save gas. With a V8 4x4 Dakota, my best average was 21mpg. Normal was 14mpg. Look up hyper milers for more driving techniques.
Ed