Liquid levels can be measured using several methods. Like capacitive, ultrasonic or pressure. Capacitive method limits its use with flammable liquids. Ultrasonic in other hand seems to be to complex for such task. Pressure method is what you need for simple measurements. In following project MPX2010 differential pressure sensor is used where on end is dipped in to liquid using tube.
using simple static liquid pressure formula its height can be estimated easily. Design is split in to two parts – transmitting and receiving. So transmitter with sensor can be placed in some radius without worrying about wires. Transmitter and receiver are simple and cheap 433Mhz RF modules. Both parts are powered by MSP430 microcontrollers from TI.