Besides the realization of several new RF systems for the new heavy-ion synchrotron SIS100 and the storage rings CR and HESR, the FAIR project also includes an upgrade of the RF systems of the existing accelerator rings such as SIS18. The SIS18 RF systems currently comprise two ferrite cavities, three broadband magnetic-alloy cavities and one bunch-compressor cavity. In addition, the LLRF system has been continuously upgraded over the past years towards the planned topology that will be implemented for all FAIR ring accelerators. One of the challenges for the SIS18 RF systems is the large RF frequency span between 400 kHz and 5.4 MHz. Although the SIS18 upgrade is still under progress, a major part of the functionality has already been successfully tested with beam in machine development experiments (MDE). This includes multi-harmonic operation such as dual-harmonic acceleration and further beam gymnastics manipulations such as bunch merging and bunch compression. Many of these features are already used in standard operation. In this contribution, the current status is illustrated and recent MDE results are presented that demonstrate the capabilities of the RF systems for FAIR