Yes, evaporation does meet the RCRA definition of treatment and is regulated as thermal treatment (Memo, Williams to Sasson; May 1, 1987 (RCRA Online #12923)).
The following document provides additional guidance on evaporation as treatment:
Memo, Shapiro to Directors, Regions 1-10, October 17, 1994 (RCRA Online #11881).
These guidance documents are available on the RCRA Online database at the following URL: https://yosemite.epa.gov/osw/rcra.nsf/how+to+use?OpenForm.
In addition, it is important to note that this guidance represents clarification of the Federal regulations. Most states are authorized to implement the Federal regulations. We recommend that you also contact your state's implementing agency to acquire additional information on evaporation and hazardous waste treatment. The Office of Resource Conservation and Recovery (ORCR) has compiled a complete list of phone numbers and waste program Web sites maintained by EPA Regional offices and state environmental agencies to help users locate the appropriate agency within their states. This compilation is found at the following URL: https://www.epa.gov/hwgenerators/links-hazardous-waste-programs-and-us-state-environmental-agencies