RV tanks have little button sized sensors that are screwed into the tank every 2 inches or so. When water is touching the button, it completes the circuit and sends a signal to the monitor. Most monitors are just a series of LEDs in a vertical line - each LED represents a button sensor on the tank. So, if the bottom 4 LEDs are lit up, you know you have 8" of water in your tank (your math will vary depending on the size of your tank and how you space out your sensors).
It's common for the sensors to go out, so any decent RV repair show should have many on hand. As for the monitor, I think it would be easy enough to build one if you didn't want to buy a ready-made unit. Check out your local RV dealership/repair shop.
HTH,
Billy