I don’t see any reason you cannot use a 7.2V battery pack to power your motors and the TReX. You should check the voltages your servo can take since it might be as low as 4.8V. If your servo’s operating voltage is less than 7.2V, you might consider a step-down regulator to reduce the voltage.
You might find this blog post helpful in determining what capacity battery pack you should use for a 30 minute runtime. You will need to figure out the combined current draw of all of the devices in your system to do that calculation.