your ENGINE running will have no effect on whether you have Shore Power, or the Generator is running - these are completely separate systems. You can even run your generator while traveling down the road, and you can run your engine while plugged into shore power, etc.
if you have a built-in Automatic Transfer Switch, and I assume you do, it will automatically switch between Shore Power and the Generator's power, after 20-30 seconds of the generator running, EVEN if you are plugged into Shore Power, and EVEN if the Engine is also running - but that's also it's job, and it chooses 'Generator power' over Shore power when there are two active power sources.
Regardless, you may hear the 'thump' of the ATS switching over after the generator is running, which is normal, just like you may hear that 'thump' when you first plug into Shore Power. The ATS is a 'box' where these wires all come together, with magnetized 'switches' to connect the various power sources.
Most Class A coaches today are built with ATS wiring, and you no longer have to be concerned about 'when' and 'what' you are using, power wise. Some older models from years ago required you to get out, go around to your shore cord, and plug your cord into a special outlet, when you needed to run the generator, since ATS switches weren't as commonplace then.
I also imagine that you can try something that may also work without having to actually have the engine RUNNING - try simply turning the ignition key to the 'ON' position, rather than fully cranking it - the slides may then be active even without the engine running.
two Campers, two Electric cars
former diesel pusher traveler