21 is pretty terrible mileage for a 2019 mid-sized.
Is this normal for a Ranger?
Yes, underpowered engine towing a big (think aerodynamic frontal area) trailer will suck the gas in, where a big v-8 truck will do better as it is not working so hard.
If you like Fords and towing, look at the F150 for 28 highway and will get much much better mpg towing, and the engine and trans is much more suitable for towing something this size.
You need a smaller trailer, or just live with the poor mpg. Research how to change your driving habits to get better mpg's too.
When I tow my cargo trailer with a similar truck to yours (cargo trailer is only 5 feet wide), I can get 16-19 mpg. It's not the weight, it's the aerodynamics that affect mpg.