Hi Reginald,
Thank you for using Syncfusion products.
We are sorry to say that currently we don't have the support "display custom strings in tool tip of RangeSlider" in our product. We have a created a new support incident and kindly follow up with the incident given below,
http://www.syncfusion.com/support/directtrac/incidents/131864
Please let us know if you need further assistance on this.
Regards,
Jessie