On many RVs the PCM is mounted to an interior surface that is not tied to ground - a fibreglass panel by the passengers feet, for example. This means that the PCM is not grounded by its mounting screws as it would be in a more normal vehicle.
Look under the RV for a loose wire that's come disconnected from the chassis or engine block. Find where the wire loom from the PCM comes through the firewall, and look for a single wire coming out of the loom heading down to the frame, or a ground point on the block.
I had this exact problem on a Ford based Airex, and as soon as I found and reconnected that loose ground, it ran fine.
Doesn't mean it's *not* a bad PCM, but eliminate the easy fix first.