Depending on engine size, gear ratio, terrain and your driving habits, 7 MPG would not be unusual and may be normal for you.
Rangers are not exactly a prime choice for towing, they are more designed for good mileage under no loads..
I have 2019 and 2020 F250s with 6.2 gas towing 7K lbs loaded and typically get low of 9.9 MPG to high of 10.2 MPG at 65 MPH.
Driving habits can make a huge difference in the mileage department, lower your expectations a bit and lighten up on the go pedal, your not going to be Mario Andretti with winning hole shots from stop lights when towing if you expect to get better fuel mileage.
My DW gets 13 MPG empty when driving and I can get 16 MPG driving, she takes the hole shots off the line and I ease on the gas..
Go light on the gas pedal when starting from a stop light, pretend there is a raw egg between your foot and the gas pedal and you don't want to break it. Let the engine and transmission work a bit easier.
Speed definitely makes a difference, 60 MPH will be much better mileage than 75 MPH (that is scary fast for towing, stopping that rig under emergency conditions at that speed may not end well for you) for mileage.
I would however be careful of going way too slow for the posted speeds. I know there are some folks on the forum that drive 58 MPH on roads posted 70 MPH.. That makes for a pretty hazardous situation when you have a large speed difference between vehicles on the same road. Around my area, roads posted 70 MPH, folks tend to be running 80MPH or a bit above that.