What size trailer hitch does yours have? If it's the smaller Class II hitch with a 1-1/4" square receiver, then most likely it's got the 3.55 axle ratio. If it's got the larger Class III hitch with the 2" receiver, then it's got the 3.73 axle ratio. The Class III towing package automatically gave you the 3.73 axle as well as a few other things.
10 MPG does seem low. Bear in mind that right now is not the opportune time to gauge your MPG, as the fuel blends and cold weather definitely kill MPG. But, even my worst tankful of E85 fuel, which does get less MPG than gas wasn't even that bad.
Your thermostat is working correctly, isn't it? If the engine is running too cold, that'll kill MPG.
You could try a different fuel supplier. Some fuels tend to deliver a little better MPG than others. I've had my best MPG on Shell fuels.
Changing out all the fluids for synthetics will help a little...Especially the differential fluids. That gear lube gets thick as tar in the cold. I'd recommend a full synthetic such as Mobil 1, Amsoil, Schaeffer, etc. in 75W140 weight in both the front and rear diffs.
Is this something that just started happening, or has it always gotten this poor of MPG?