The basics are quite simple.
The motor generates a certain torque
N and a certain power
P at a given RPM. Further more, the relation between power and torque is:
P = C * N * RPM
C is a constant to convert all that odd units. For
P in SI units, it is
C = pi / 30
Neglecting any losses, Power is conserved from the motor to the wheels so you can say
C * N_motor * RPM_motor = C* N_wheel * RPM_wheel
N_wheel = N_motor * RPM_motor / RPM_wheel
The RPM of the wheel can easily be derived from speed and the rolling circumference of the wheel
R_wheel. (Keep in mind, the rolling circumference is smaller than the geometrical circumference, as the tire is flexible.
Since you are more interested in the tractive force
F_wheel, it is
N_wheel = F_wheel * R_wheel
F_wheel = N_motor * RPM_motor / (RPM_wheel * R_wheel)
As it's also
v = pi * RPM_wheel * R_wheel / 30 (velocity in m/s) you can write
F_wheel = N_motor * RPM_motor * pi / (v * 30)
This means, if you really have the torque the motor currently delivers, the RPM of the motor and the speed of the truck, you can calculate the total force applied to the street.
It's strange that gear ratios don't appear, but they are hidden in the ratio
RPM_motor / RPM_wheel or
RPM_motor / v.
In reality, there are lots of non neglible losses, as Paulster2 wrote in his comment.
Every bearing and every gear wheel has some friction, taking away some torque and power. If this torque is constant, the power loss will be linear to the RPM of this part, but usually, torque will increase with RPM, so the power loss grows even faster with RPM.
This means the loss is not constant, it varies with RPM and gear!
One interesting fact: A clutch transfers power / torque by friction. For a slipping clutch, torque is the same on both shafts, but power is lost due to the difference in RPM...
And to make it clear again: You do need the measured torque from the motor, max torque at given RPM is not what you need.