I assume that by "holding tank" that you have a system which drains from the septic tank into a holding tank. There is a pump in this tank which then pumps the water into the elevated drainfield. If this is true, then your holding tank should have an alarm already installed. If it is not working, an electrical contractor or a septic system installer should be called to repair ir or to install one. It is essential that this holding tank have a high water alarm on it to prevent backups into the house. This should not be terribly expensive. 

If this is not how your system works, then please give me a call to discuss further so I can understand better what your problem is. If you need a high water alarm, you should not put off having one installed or repairing the one already installed.