How to do low pass filtering on a differential signal
how would I do a simple RC low pass filter on this differential
signal?

The simplest method: -

Source. This circuit only filters differential signals; if there is an in-phase signal (or interference) on both lines (called a common-mode signal/noise) it provides no filtering to it.
A more complex but effective method: -

Modified from this Source. This filter provides differential and common mode filtering. C2 and C3 are the extras but, be aware that if the differential signal is required to remain highly &quot;balanced&quot; in nature, tolerances on C2 and C3 can displace the perfect symmetry at higher frequencies.