The Maestro already measures the VIN voltage. It also has an extra thermistor input (4 in total), so you can already measure chamber temperature.
To control your LEDs, if it is just one single colour LED string then if you have a spare extruder heater output or fan output, you can use that.
If you do need to communicate with another board, then I suggest you use either the I2C pins or the UART pins on the expansion connector.
HTH David