RS485 voltage levels
You can find the required voltage difference at the sending and receiving ends in the RS485 standard. As a second best, read the specs of some RS485 transceiver.

What is probably the problem in your project (but it is difficult to reconstruct the original signal after a half-garbling filter) is that the common of your sender and receiver differ too much. That is, there are two requirements for successful RS485 communication:


the voltage difference between D+ and D- must be OK. This is probably not your problem.
the voltage difference between D+ and ground, and D- and ground must be in the range acceptable by the receiver. If they are not, you have a ground potential difference problem. To solve it, you can either reduce the difference, or find a receiver that can tolerate a higher difference.


The voltage levels for RS485, from The RS485 design guid :



Note that this picture is about the differential voltages, it omits the ground connection.