I would think it is more likely that you didn't fill up the tank
completely when you measured the amount of fuel used.

Jeff

Why would you keep the windows up and recirculate the air inside the cabin?
You should bring in fresh air from outside, especially in the rain. I'd go
so far as to say you should run the AC when it is raining, just to prevent
the windows from getting fogged up.

The trailer is weight and an adverse affect to the aerodynamics. You should
through out the numbers you get while dragging it around.

You have only had the car for a month, you need more time to figure out what
you are getting. Let's assume in a month that you drive 1200 miles, and only
get 20MPG. This means you bought 60 gallons of gas, the car holds 20, so you
filled it three times. This is not enough to figure out what you are really
getting. If you drove a lot in the city pulling the trailer, and you press
the gas with gusto, then your numbers for that tank will be way down. If you
get on the interstate and drive for 300 miles and keep a reasonable speed,
your numbers will be way up. It so happens that you gave both extremes in
your question ...

I doubt the odometer is off, but you can check it against a measured mile
with a stop watch and a constant speed. Measure the mile with the stop
watch, divide 3600 by the measurement, the result will be your speed. This
checks the speedometer.

You check the odometer by simply looking at it at the start of the measured
mile, and again at the end of the mile, and make sure it has ticked off one.

A Measured Mile is one where the highway department installs a small placard
on the shoulder of the highway (interstate, state highway, or county road)
to notate the location. The highway people use these markers to note the
location of maintenance that might be needed, or perhaps to log the location
of an accident -- pot hole filed at marker 27.5, accident on NB I-45 at
marker 67, that sort of thinig. As you are driving along the highway, take
the opportunity to look at the many different kinds of sign posts along the
shoulder. You will notice a pattern start to emerge. These signs, when
posted, will be within a few feet of the right place. There will be some
that are missing, but the ones that are there will be in the right place for
your measurements. The signs on the freeway should be at half-mile
intervals, and call boxes are generally installed where a mile marker would
be expected. Since you will want to measure a mile, or several of them, at a
constant speed, you probably want a freeway mile not a twisty mountain road
mile -- which would preclude most county roads and some state highways.

It is possible that the odometer is off, but not inconsistently.  In other
words, if it is off by .1 miles per mile at 55 MPH, it will always be off by
that amount, not more on some days and less on others.

A lot of things affect fuel economy.  Obviously, pulling a trailer adds
weight and drag, which have an adverse effect of fuel economy.  Having
something on the roof rack also adds weight and drag.

Stopping, idling, and accelerating have an adverse effect, which is why cars
generally get worse fuel economy in the city than on the highway.  Driving
at a steady speed means that the transmission is in overdrive with the
torque converter locked so fuel economy is improved.

Using the AC has a slight but probably not measurable adverse effect on fuel
economy, perhaps 1 MPG if it is not used for an entire tank of fuel.

Driving with the windows up usually reduces drag, which has a positive
effect.

Cool, moist air is denser than hot, dry air so it mixes with fuel more
completely.

Engines operate more efficiently when completely warmed up, so if the trip
is long and the engine stays warmed up, you will get better fuel economy
than the same distance made up of many short trips where the engine has to
warm up each time.

Fuel pumps click off at different times, so there is a pretty big variance
between pumps.  By the way, trying to top off the fuel tank until the fuel
is at the top of the fuel filler neck is not good for the evaporative
emissions control system and can contaminate the charcoal canister.  When
the fuel pump clicks off, you can round up to the nearest quarter or maybe
dollar, but topping off after that is not a good idea.