Are you saying there is no voltage difference between neutral and hot? That's what you see if they are tied together.
Or are you comparing each to ground? Neutral and ground will not be tied together in your RV. The RV is treated as portable appliance and a ground-neutral tie at that point would be considered a fault. Neutral and ground come together back at the power source, i.e. the post. A fault could be where you plugged in, your power cord, or where the power line comes into the box. Most of the time I've had problems, the problem was where I plugged in. But excessive resistance on one line or the other could read as a voltage when measuring neutral to ground, while plugged in.
I've also had ground faults occur in the GFCI outlet itself, not enough connection to carry a load, but enough to cause GFCI problems.