What would cause a square wave to become rounded?
What you are seeing are several harmonics of the square wave being attenuated. As you know, a square wave can be represented by an infinite series of sine waves whose frequencies are multiples of the square wave's fundamental.



Image source: Fourier sine series: square wave, Math 331, Fall 2017, Lecture 1, (c) Victor Matveev

In real life there is no such thing as a true square wave. Every "square wave" is actually closer to a trapezoid in shape with sloped edges. The slope of these edges can be used to determine how high up the frequency content of your "square" wave goes. For example, if you have a square wave with edges that move from 0V to 3.3V in 1ns, you probably have some frequency content all the way up around 1GHz. As you lose frequency content transmitting the square wave, it starts to become closer and closer to a sine wave.

The next question is, what causes you to lose frequency content:


The cable might have a relatively "lossy" material as its dielectric. It has more impedance at higher frequencies and therefore attenuates harmonics of the signal more at those frequencies, forming a low-pass filter after a long enough distance. 1000ft is a very long distance. P-spice likely won't take this into account unless you have an S-parameter model of your cable or a lossy transmission line model of the cable.
The cable has some capacitance between the conductors and the outer shield (and between the conductors themselves). It is likely in the low fF or pF per foot, but after 1000 feet that's not a trivial amount of capacitance. The combination of the cable capacitance and your driver's output impedance (120 ohms) will create a low-pass filter. The cable manufacturer should give you the cable capacitance in the cable datasheet. Calculate the corner frequency (120Ohms * The total cable capacitance) of the resulting low-pass filter with 1000ft of cable and you might find something interesting. Is P-Spice taking this into account?


In my opinion, it is probably more likely you have a cable capacitance problem, especially since you say that it takes a while for the signal to return back to 0V.

Additional information that would be useful:


What is the rise and fall time of your signal? That tells us the frequency content of your signal.
A scope shot of your signal at the input and signal at the output of your cable.
The part number of the cable you are using.