The payload formats are defined in main.h. eg.
uint8_t humidity; // humidity percent
float temperature; // temperature in degrees C
uint16_t voltage; // in millivolts
uint16_t pressure; // hPa
uint16_t gas; // kOhms
float altitude; // meters
For the moment, we suggest adapting the above to meet your UV structure. Once we've completed and tested the revised UV code, it'll be posted to GitHub and yours can be replaced with the TTN-compatible structure.