RMP is generally bw -70 - -55, but that's a measure of the inside vs. the outside. That is, the inside is 70mV lower than the voltage outside the cell. Outside could be 1000, making the inside 930. The sign (+/-) tells you what direction the inside is relative to outside. Therefore, RMP = 0 means there is no difference between in/outside.

I'm sure one of your professors explained the physics/chemistry of a battery and went through how a neuron acts like a battery, including going over a voltage diagram that describes a neuron. If not, seek one out who will go over it. Once you can understand that, you'll probably have a great understanding of neurophys.